Invar alloy and preparation method thereof

Abstract

The invention discloses an invar alloy and a preparation method thereof, belongs to the technical field of invar alloys, and solves the problem that the mechanical strength, the low expansion characteristic and the corrosion resistance of the invar alloy cannot be improved at the same time in the prior art. The invar alloy comprises the following components in percentage by mass: 35.0%-37.0% of Ni, less than or equal to 0.03% of C, less than or equal to 0.30% of Si, 0.20%-0.60% of Mn, 0.10%-0.30% of Zr, 0.01%-0.03% of Ce and the balance of Fe and inevitable impurities. The invar alloy is excellent in comprehensive performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of invar alloy, in particular to an invar alloy and a preparation method thereof. BACKGROUND

[0002] 4J36 invar alloy as a kind of alloy material with unique performance, since the advent in many fields has been widely used. The core characteristics of 4J36 invar alloy is to have very low thermal expansion coefficient in a certain temperature range, for example, in the range of 20 DEG C to 100 DEG C, the linear expansion coefficient is about 1.5*10 -6 / ℃. This low expansion characteristic is due to its special crystal structure and magnetic interaction. With its unique low expansion characteristics, 4J36 invar alloy occupies an irreplaceable position in many fields with strict size accuracy requirements.

[0003] However, with the increasing complexity of application scenarios, higher requirements are put forward for the performance of 4J36 invar alloy. On the one hand, in the structure application of bearing large load, its originally relatively low mechanical strength limits its further popularization and use. On the other hand, in some special working environment, such as humid marine environment or industrial environment with corrosive medium, the corrosion resistance of 4J36 invar alloy needs to be further improved to prolong its service life and ensure the stable operation of equipment. In addition, although it has a low thermal expansion coefficient, in the ultra-precision application scenario, it is still expected to further reduce its thermal expansion coefficient to meet the higher precision requirement.

[0004] At present, some researches have been carried out on the performance improvement of 4J36 invar alloy, but these methods have limited effect on the comprehensive improvement of the mechanical strength, low expansion characteristics and corrosion resistance of 4J36 invar alloy, especially it is difficult to realize the synergistic optimization and synchronous improvement of the above performances. SUMMARY

[0005] In view of the above situation, the present application aims to provide an invar alloy and a preparation method thereof, which solve the technical problem that the mechanical strength, low expansion characteristics and corrosion resistance of the existing invar alloy cannot be improved at the same time.

[0006] The purpose of the present application is mainly realized by the following technical scheme:

[0007] The present application provides an invar alloy, the components of the invar alloy include, in terms of mass percentage: Ni: 35.0%~37.0%, C:≤0.03%, Si:≤0.30%, Mn:0.20%~0.60%, Zr:0.10%~0.30%, Ce:0.01%~0.03%, the balance is Fe and inevitable impurities.

[0008] Further, the microstructure of the invar alloy comprises equiaxed single-phase austenite and second-phase particles dispersedly distributed in the austenite matrix, and the second-phase particles mainly comprise Ni7Zr2 intermetallic compound.

[0009] Further, the second-phase particles further comprise Ce2O2S.

[0010] Further, the invar alloy has a thermal expansion coefficient α ≤ 0.61 × 10 -6 / ℃ in a temperature range of 20-100℃, a yield strength of 400 MPa or higher, and a tensile strength of 500 MPa or higher.

[0011] The application further provides a preparation method of the invar alloy, comprising the following steps:

[0012] Step 1: iron material, nickel material, manganese material, zirconium material and cerium material are weighed according to mass percentage, the iron material and the nickel material are loaded into a crucible, and the manganese material, the zirconium material and the cerium material are separately loaded into material bins;

[0013] Step 2: smelting of the iron material and the nickel material is performed, after the iron material and the nickel material are completely melted, the manganese material is added into a molten pool for heat preservation, then the cerium material is added for heat preservation, and finally the zirconium material is added for heat preservation;

[0014] Step 3: a cast ingot is obtained by casting;

[0015] Step 4: the cast ingot is subjected to homogenization treatment, and then is forged to obtain a forged blank;

[0016] Step 5: the forged blank is subjected to solid solution treatment and aging treatment to obtain the invar alloy.

[0017] Further, in step 2, the manganese material is added into the molten pool for heat preservation for 5-10 min, and then the cerium material is added for heat preservation for 5-10 min.

[0018] Further, in step 3, the casting temperature is 1500-1550℃.

[0019] Further, in step 4, the homogenization treatment comprises heating the cast ingot to 1180-1210℃ for 1-2 h.

[0020] Further, in step 4, the initial forging temperature is controlled to be 1050-1200℃, and the final forging temperature is controlled to be 850-950℃.

[0021] Further, in step 5, the solid solution treatment and the aging treatment comprise water quenching after heat preservation at 830-850℃ for 1-2 h, and then air cooling after heat preservation at 305-325℃ for 1-2 h.

[0022] Compared with the prior art, the application can at least achieve one of the following beneficial effects:

[0023] The invar alloy of the present application can realize significant improvement of mechanical strength, effective reduction of thermal expansion coefficient and enhancement of corrosion resistance by precisely controlling the content of elements such as Ni, Mn, Zr, Ce, etc., thereby meeting the stringent requirements of modern industry for high-performance invar alloy materials.

[0024] In the preparation method of the invar alloy of the present application, the adding sequence of each raw material is precisely controlled, and the process parameters of each step are precisely controlled, so as to ensure that the microstructure of the invar alloy is uniform and fine equiaxed single-phase austenite (FCC) with a grain size in the range of 20-40 μm, and there is no enrichment of harmful impurities such as P and S; in the austenite matrix, second-phase particles are dispersedly distributed, the second-phase particles are mainly Ni7Zr2 intermetallic compounds, the precipitated phase is mainly spherical or spherical-like, and the size is concentrated in 50-200 nm; in addition, there is a small amount of Ce composite inclusions (such as Ce2O2S); there is also a proper amount of dislocation structure in the alloy, and the dislocation density is about 2.0×10 14 -2.5×10 14 m -2 . Further, a good match of mechanical strength, low expansion characteristics and corrosion resistance is ensured.

[0025] The invar alloy of the present application has excellent comprehensive performance, and the thermal expansion coefficient α is ≤0.61×10 -6 / ℃, for example 0.58×10 -6 -0.61×10 -6 / ℃, for example the low expansion performance is outstanding; the mechanical properties are balanced and reliable, the yield strength is above 400 MPa, for example 405-412 MPa, the tensile strength is above 500 MPa, for example 504-510 MPa, the elongation after fracture is more than 30%, for example 31%-34%; in addition, the alloy also exhibits excellent corrosion resistance, and the corrosion current density is less than 9 μA·cm -2 , for example 8.44-8.99 μA·cm -2 .

[0026] Other features and advantages of the present application will be set forth in the following description of the application, and in part will become apparent to those skilled in the art upon examination of the following or can be learned by practice of the application. The objects and other advantages of the application can be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. BRIEF DESCRIPTION OF DRAWINGS

[0027] The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiments of the application and together with the description serve to explain the principles of the application. In the drawings:

[0028] Figure 1The thermal expansion coefficient curve of the alloy prepared for the inventive example 1 and the comparative example 1;

[0029] Figure 2 The room temperature tensile stress-strain curve of the alloy prepared for the inventive example 1 and the comparative example 1;

[0030] Figure 3 The potentiodynamic polarization curve of the alloy prepared for the inventive example 1 and the comparative example 1. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present application will be described in detail below with reference to the drawings, wherein the drawings constitute a part of the present application and serve to explain the principles of the embodiments of the present application together with the embodiments of the present application.

[0032] The present application provides an invar alloy, the components of the invar alloy include, in terms of mass percentage: Ni: 35.0%~37.0%, C:≤0.03%, Si:≤0.30%, Mn:0.20%~0.60%, Zr:0.10%~0.30%, Ce:0.01%~0.03%, the balance is Fe and inevitable impurities.

[0033] The effects of the components contained in the present application and the selection of the amount are described in detail below.

[0034] Ni is a key element that determines the core performance of the invar alloy "low thermal expansion coefficient", its core function is to form an ordered Fe-Ni alloy phase with Fe, the magnetostriction effect of the phase can effectively offset the thermal expansion caused by the temperature rise, and then give the alloy a very low expansion coefficient; if the content of Ni is too low, the proportion of Fe-Ni ordered phase is insufficient, the magnetostriction effect cannot fully offset the thermal expansion, resulting in a significant increase in the expansion coefficient; if the content of Ni is too high, it will destroy the stability of the Fe-Ni ordered phase, which also causes the expansion coefficient to rise. Therefore, the content of Ni is accurately controlled at 35.0%~37.0% to ensure the core performance of low expansion of the alloy.

[0035] Si is a weak ferrite forming element, and excessive content (>0.30%) will have a significant deterioration effect on the overall performance of the alloy: on the one hand, it will destroy the structural stability of the Fe-Ni ordered phase, directly leading to an increase in the thermal expansion coefficient of the alloy, and then weakening its core low expansion performance; on the other hand, excessive Si will react with Fe and Ni to form brittle intermetallic compounds such as Fe-Si and Ni-Si, resulting in a significant decrease in room temperature elongation and a significant deterioration in toughness of the alloy. Therefore, the content of Si is strictly controlled at ≤0.30%.

[0036] Mn has multiple effects in Invar alloy: on the one hand, as a deoxidizing and desulfurizing agent, it can combine with O and S to form stable inclusions such as MnO and MnS, thereby reducing the damage of harmful impurities such as oxides and sulfides to the performance of the alloy and achieving matrix purification; on the other hand, it can improve the hot workability of the alloy and reduce the hot brittleness sensitivity, thereby ensuring that the alloy is not prone to cracking during hot rolling, forging and other hot working processes and optimizing the workability; at the same time, a small amount of Mn can be dissolved in the Fe-Ni matrix, slightly increasing the strength of the alloy while having little effect on the core low expansion performance, thereby playing a mild strengthening role. However, the content of Mn needs to be accurately controlled: if the content is too low (<0.20%), the deoxidizing and desulfurizing effect is insufficient, the content of O and S impurities in the alloy increases, harmful inclusions such as FeO and FeS are easily formed, the alloy is prone to cracking during hot working, and the improvement in workability is not obvious; if the content is too high (>0.60%), the Curie temperature of the alloy decreases, the temperature range of the low expansion performance is reduced, the magnetic properties of the alloy are enhanced, and the increase in hardenability leads to the formation of martensite during cooling, resulting in increased hardness, decreased plasticity and increased thermal expansion coefficient. Taking all factors into consideration, the content of Mn is controlled to be 0.20%-0.60% in the present application.

[0037] Zr is one of the key elements that synergize with Ce in the present application. Zr can react with Ni in the alloy matrix to form fine and dispersed Ni-Zr intermetallic compounds. These precipitates can effectively pin grain boundaries, inhibit the migration of grain boundaries and subgrain boundaries and the movement of dislocations during stress or heating, significantly refining the grains and simultaneously achieving grain boundary strengthening and dislocation strengthening. If the content is too low (<0.10%), the amount of Ni-Zr precipitates is insufficient, and the effects of grain refinement and strengthening are not obvious, which cannot fully play its role in performance improvement; if the content is too high (>0.30%), a large amount of Ni-Zr intermetallic compounds will be formed, which will not only lead to a significant decrease in the toughness of the alloy and cracking during cold working, but also consume Ni elements in the matrix, making it deviate from the optimal content range for ensuring low expansion performance, resulting in a significant increase in the thermal expansion coefficient. In addition, the micro-battery effect between the Ni-Zr intermetallic compounds and the matrix will significantly reduce the corrosion resistance of the alloy. Taking all factors into consideration, the content of Zr is controlled to be 0.10%-0.30% in the present application.

[0038] In the alloy system of the present application, Ce and Zr synergistically play an irreplaceable role in performance improvement. Ce preferentially adsorbs and combines with harmful impurities such as sulfur (S), phosphorus (P), oxygen (O) and the like at the grain boundaries, effectively purifying the grain boundaries, significantly inhibiting impurity segregation, thereby relieving the problem of grain boundary embrittlement caused thereby. In addition, the stable inclusions formed by Ce and impurities can act as heterogeneous nucleation cores to promote the nucleation of austenite grains during solidification or heat treatment and inhibit their excessive growth, further enhancing the grain refinement effect. This mechanism, together with the Ni-Zr precipitates formed by Zr, forms a synergistic refinement system of "grain boundary purification-precipitate pinning". The content of Ce needs to be strictly controlled to ensure the synergistic effect: if the content is too low (<0.01%), the grain boundary purification effect is insufficient, impurities are still prone to segregate at the grain boundaries, the number of heterogeneous nucleation points is limited, effective grain refinement is difficult to achieve, and full synergy with Zr cannot be achieved; if the content is too high (>0.03%), coarse Ce-containing oxysulfides are easily generated, which become stress concentration sources, leading to a decrease in mechanical properties and the easy induction of cracks during processing, and excessive Ce may also form brittle intermetallic compounds, weakening the overall performance of the alloy. Taking all factors into consideration, the content of Ce in the present application is controlled to be 0.01%-0.03%.

[0039] Specifically, in the present application, the combined micro-alloying treatment of Zr and Ce, combined with the process regulation of "forging plastic deformation + solid solution-aging heat treatment", realizes the effective improvement of the comprehensive performance of invar alloy. The synergistic strengthening mechanism of each performance dimension is as follows: in terms of strength improvement, forging plastic deformation introduces a large number of dislocations into the alloy, and subsequent solid solution-aging heat treatment realizes dislocation regulation through solute atom segregation and second phase precipitation. The fine and dispersed Ni-Zr intermetallic compounds formed by the reaction of Zr and Ni, together with the Ce-containing inclusions induced by Ce, form a "double second phase synergistic hindering system", which collectively produces a strong hindering effect on the movement of internal dislocations, significantly reduces the dislocation slip rate, and greatly enhances the dislocation strengthening effect; at the same time, the grain boundary purification effect of Ce reduces the weakening of harmful impurities on the grain boundary bonding force, and the Zr precipitates realize efficient grain boundary pinning, forming a synergistic effect of grain boundary strengthening, and finally realizing the significant improvement of the strength of invar alloy. In terms of low expansion performance improvement, Zr and Ce form a double synergistic regulation. On the one hand, Ce reduces the interference of harmful impurities such as S and P on the arrangement of grain boundary atoms through grain boundary purification, and on the other hand, it significantly refines the austenite grains through heterogeneous nucleation, laying a fine-grained foundation for low-expansion structure; Zr forms fine and dispersed Ni-Zr intermetallic compounds with the matrix Ni, which can effectively constrain the thermal vibration amplitude of the crystal lattice, enhance the interatomic bonding force, and form a synergy from the two dimensions of "grain boundary optimization-grain refinement" and "lattice constraint", ultimately realizing that the alloy of the present application has a low expansion coefficient of α≤0.6×10 -6The low expansion performance is less than or equal to 0.61*10-6 / ℃. In the aspect of improving the corrosion resistance, when Zr and Ce are added together, the sensitivity of pitting corrosion initiation is effectively reduced, and the corrosion process is inhibited from the corrosion source; at the same time, the substrate purification and second phase precipitation of Ce and Zr can synergistically optimize the structure of the passivation film on the surface of the alloy, significantly improve the denseness of the passivation film, and enhance the protection ability of the passivation film to the substrate, so that the overall corrosion resistance of the alloy is improved under the double synergistic effect.

[0040] Specifically, the inevitable impurities in the above-described Invar alloy include, by mass percentage: P: ≤0.02%, S: ≤0.02%, O: ≤0.0015.

[0041] Specifically, in order to further improve the comprehensive performance of the above-described Invar alloy, the components of the above-described Invar alloy include, by mass percentage: Ni: 35.0%-36.98%, C: 0.002%-0.004%, Si: ≤0.30%, Mn: 0.21%-0.59%, Zr: 0.10%-0.30%, Ce: 0.01%-0.03%, and the balance is Fe and inevitable impurities.

[0042] Specifically, the comprehensive performance of the above-described Invar alloy is excellent, and the thermal expansion coefficient α is less than or equal to 0.61*10-6 / ℃ in the temperature range of 20-100℃, for example, 0.58*10-6 / ℃-0.61*10-6 / ℃, for example, the low expansion performance is outstanding; the mechanical properties are balanced and reliable, the yield strength is more than 400 MPa, for example, 405-412 MPa, the tensile strength is more than 500 MPa, for example, 504-510 MPa, and the elongation after fracture is more than 30%, for example, 31%-34%; in addition, the alloy also exhibits excellent corrosion resistance, and the corrosion current density is less than 9 μA·cm-2, for example, 8.44-8.99 μA·cm-2. -6 -6 -6 -2 -2 .

[0043] On the other hand, the present application also provides a preparation method of the Invar alloy, comprising the following steps:

[0044] Step 1, iron material, nickel material, manganese material, zirconium material and cerium material are weighed according to mass percentage, the iron material and the nickel material are loaded into a crucible, and the manganese material, the zirconium material and the cerium material are separately placed into material bins;

[0045] ​​​​Step 2, smelting of iron and nickel materials is performed, after the iron and nickel materials are melted, the manganese material is added to the molten pool for 5-10 min, the uniform solid solution of manganese element is controlled through the holding time, and local component segregation caused by insufficient dissolution is avoided; then the cerium material is added for 5-10 min, in this process, the residual gas and inclusions in the molten pool are removed through the purification effect of the cerium element, and the performance of the alloy is improved by using the micro-alloying effect of cerium; then zirconium material is added for holding, the addition of zirconium element can form stable intermetallic compounds, improve the mechanical properties of the alloy, and improve the low expansion performance, thereby ensuring the size stability and structural reliability of the alloy during use;

[0046] Step 3, casting to obtain an ingot;

[0047] Step 4, homogenizing treatment is performed on the ingot, and then forging is performed to obtain a forged blank;

[0048] Step 5, solid solution treatment and aging treatment are performed on the forged blank to obtain the invar alloy.

[0049] Specifically, in the above step 1, the iron material and the nickel material are loaded into the crucible in a tight bottom and loose top manner. The tightly packed iron material at the bottom can form a stable heat conduction framework, effectively avoiding the risk of molten spray or "bridge" caused by local premature collapse, and ensuring a stable and controllable initial melting process; at the same time, the loose arrangement of the nickel material at the top can gradually melt and fall after the lower iron material is basically melted, which can not only reduce the high-temperature volatilization loss of nickel element, but also promote the uniform mixing of alloy components through the convection effect of the melt, thereby significantly reducing macro-segregation; in addition, the gradient melting sequence formed by this loading method enables the inductive electromagnetic field energy to act on the high-melting-point material at the bottom, which not only improves the thermal energy utilization efficiency, but also shortens the overall smelting cycle, and has both process safety and economy.

[0050] Specifically, in the above step 1, the crucible is made of magnesium aluminate spinel.

[0051] Specifically, in the above step 2, a vacuum induction furnace is used to smelt under a high-purity argon protective atmosphere to prevent spatter during the melting process of the raw materials.

[0052] Specifically, in the above step 2, the cerium material is added first after the manganese material is held, and then the zirconium material is added, which can ensure that Ce reacts with O and S in the molten pool to form fine inclusions containing Ce, thereby fully playing the role of purifying the matrix and removing harmful impurity elements such as O and S; at the same time, it avoids the formation of coarse and hard brittle zirconium oxysulfide inclusions when zirconium is added first, or the formation of Ce-Zr brittle phases when zirconium is added at the same time as cerium, thereby ensuring that zirconium is fully solid-solved in a low-impurity environment, providing conditions for the subsequent precipitation of 50-200 nm Ni-Zr strengthening phases, and finally realizing the optimization of composition uniformity and performance.

[0053] Specifically, in the step 3, the excessively high casting temperature is easy to cause coarse grains, aggravate element burning and increase the tendency of shrinkage; the excessively low temperature is easy to cause insufficient fluidity, difficult floating of inclusions and cold shut defects. Therefore, the casting temperature is controlled to be 1500-1550°C, for example, 1500°C, 1510°C, 1520°C, 1530°C, 1540°C or 1550°C.

[0054] Specifically, in the step 3, after the ingot is completely cooled, the furnace cover is opened and the ingot is taken out.

[0055] Specifically, in the step 4, the excessively high temperature of the homogenization treatment is easy to cause abnormal grain growth, local overburning and even initial melting, and increase the risk of cracking in the subsequent forging process. The excessively low temperature is difficult to effectively eliminate the dendritic segregation and composition non-uniformity in the ingot, and difficult to realize the full diffusion of alloy elements, and the homogenization effect is poor. The excessively long holding time not only increases the energy consumption and production cycle, but also may be accompanied by grain coarsening phenomenon, and reduces the processing performance of the material; the excessively short holding time is difficult to fully diffuse the alloy elements, and difficult to completely eliminate the micro-segregation formed in the casting process, and difficult to achieve the expected effect of the homogenization treatment. Therefore, the homogenization treatment includes: heating the ingot to 1180-1210°C and holding for 1-2h. By controlling the homogenization temperature and time, the dendritic segregation in the ingot is eliminated, and the alloy element distribution tends to be consistent, so as to provide good organizational conditions for plastic deformation in the forging process.

[0056] Specifically, in the step 4, the excessively high initial forging temperature is easy to cause grain coarsening and even overburning, and cause plasticity deterioration and internal cracks; the excessively low temperature significantly increases the deformation resistance, inhibits dynamic recrystallization, and easily causes the forging to crack. The excessively high final forging temperature is easy to cause static recrystallization and grain growth during the cooling process after forging, and cannot retain the refined organizational effect produced by forging, and affects the mechanical properties of the final product; the excessively low final forging temperature sharply reduces the plasticity of the material and aggravates the hardening degree, and easily causes edge cracks, corner cracks and other problems in the later forging stage, and the excessively large deformation resistance may exceed the carrying capacity of the equipment, and increases the production risk. Therefore, the initial forging temperature is controlled to be 1050-1200°C, for example, 1050°C, 1100°C, 1150°C or 1200°C; and the final forging temperature is controlled to be 850-950°C, for example, 850°C, 900°C or 950°C.

[0057] Specifically, in the step 4, the reasonable forging ratio can break the coarse grains and defects in the casting structure through sufficient plastic deformation, promote dynamic recrystallization to refine the grains, and improve the density of the forging blank; if the compression ratio is too small, the deformation amount is insufficient, and the casting defects cannot be completely eliminated, and the organizational uniformity is poor; if the compression ratio is too large, stress concentration is easily caused by local excessive deformation, and the cracking risk is increased. Therefore, the forging ratio is controlled to be 2.5-5.5.

[0058] Specifically, in the above step 5, the solution treatment and aging treatment include: water quenching after holding at 830-850℃ for 1-2h, and then air cooling after holding at 305-325℃ for 1-2h.

[0059] It should be noted that, in the above step 5, if the quenching holding temperature of the solution treatment is too high, the alloy grains will grow excessively, reducing the mechanical properties and dimensional stability of the material; if the temperature is too low, the solid solution in the alloy will not be fully dissolved, and subsequent aging treatment will be difficult to optimize the performance, thus failing to meet the target strength and hardness requirements, therefore, the quenching holding temperature is controlled to be 830-850℃, such as 830℃, 840℃, 850℃. If the aging treatment holding temperature is too high, the precipitated phase will grow excessively, resulting in a decrease in material performance; if the temperature is too low, the nucleation and growth of the precipitated phase will be insufficient, resulting in insufficient material strength and hardness, therefore, the aging holding temperature is controlled to be 305-325℃, such as 305℃, 310℃, 320℃, 325℃. By precisely controlling the temperature, holding time and cooling method of this step, the characteristics of the microstructure are adjusted, achieving the synergistic matching of low expansion coefficient, excellent mechanical properties and corrosion resistance of the invar alloy.

[0060] Specifically, the microstructure of the above invar alloy is a uniform and fine equiaxed single-phase austenite (FCC) structure, with a grain size in the range of 20-40μm, fine grain size, and equiaxed grain morphology without obvious preferred orientation, and no enrichment of harmful impurities such as P and S; in the austenite matrix, there are dispersed second phase particles, mainly Ni7Zr2 intermetallic compounds, which are mainly spherical or spherical, with a size of 50-200nm; in addition, there are a small amount of Ce complex inclusions (such as Ce2O2S); there are also appropriate dislocation structures inside the alloy, with a dislocation density of about 2.0×10 14 ~2.5×10 14 m -2 .

[0061] The invar alloy of the present application can significantly improve the mechanical strength of the alloy, effectively reduce the thermal expansion coefficient, and enhance the corrosion resistance by precisely controlling the content of elements such as Ni, Mn, Zr, Ce, etc., thus meeting the stringent requirements of modern industry for high-performance invar alloy materials.

[0062] The preparation method of the Invar alloy of the present application controls the adding sequence of each raw material accurately and controls the process parameters of each step accurately, so as to ensure that the microstructure of the Invar alloy is uniform and fine equiaxed single-phase austenite (FCC) with a grain size in the range of 20-40 μm, and there is no enrichment of harmful impurities such as P and S; in the austenite matrix, second-phase particles are dispersedly distributed, the second-phase particles are mainly Ni7Zr2 intermetallic compounds, the precipitated phase is mainly spherical or spherical-like, and the size is concentrated in 50-200 nm; in addition, there is a small amount of Ce composite inclusions (such as Ce2O2S); there is also a proper amount of dislocation structure in the alloy, and the dislocation density is about 2.0×10 14 ~2.5×10 14 m -2 . Further, a good match of mechanical strength, low expansion characteristics and corrosion resistance is ensured.

[0063] The Invar alloy of the present application has excellent comprehensive performance, and the thermal expansion coefficient α is ≤0.61×10 -6 / ℃ in the temperature range of 20-100℃, for example 0.58×10 -6 -0.61×10 -6 / ℃, for example the low expansion performance is outstanding; the mechanical properties are balanced and reliable, the yield strength is above 400 MPa, for example 405-412 MPa, the tensile strength is above 500 MPa, for example 504-510 MPa, the elongation after fracture is more than 30%, for example 31%-34%; in addition, the alloy also exhibits excellent corrosion resistance, and the corrosion current density is less than 9 μA·cm -2 , for example 8.44-8.99 μA·cm -2 .

[0064] The advantages of accurate control of the composition and process parameters of the present application are demonstrated below with specific examples and comparative examples.

[0065] Examples 1-6 of the present application provide an Invar alloy and a preparation method thereof, and the chemical composition of the Invar alloy of Examples 1-4 is shown in Table 1. The examples of the present application use a vacuum induction furnace to smelt the alloy under an argon protective atmosphere, and the raw materials used for smelting include industrial pure iron, nickel plate, electrolytic manganese sheet, sponge zirconium and cerium metal.

[0066] The preparation method of Example 1 includes:

[0067] (1) Prepare raw materials: accurately weigh the raw materials according to the mass percentage, and adopt the "tight bottom and loose top" charging method to charge the industrial pure iron and nickel plate into the magnesium aluminate spinel crucible, and separately put the electrolytic manganese, sponge zirconium and cerium metal into the material bin;

[0068] (2) Smelting: Power on and heat up while starting the vacuum pump to evacuate to below 5.0 Pa, then inject 20 kPa high-purity argon gas. After the iron and nickel materials are melted, add the electrolytic manganese in the silo into the molten pool to melt.

[0069] (3) Microalloying: After 5 minutes, add metallic cerium first, wait 5 minutes, and then add sponge zirconium;

[0070] (4) Casting: Control the casting temperature at around 1520℃, complete the casting, and after the ingot has completely cooled, open the furnace cover and take out the ingot.

[0071] (5) After homogenizing the 105cm diameter ingot at 1200℃ for 1h, the billet was forged. The initial forging temperature and the final forging temperature were set to 1150℃ and 900℃ respectively, and the diameter after forging was 60cm.

[0072] (6) The forging billet is held at 840℃ for 1 hour and then water quenched, followed by annealing at 315℃ for 1 hour and finally air-cooled.

[0073] The preparation method of Example 2 is the same as that of Example 1, except that:

[0074] (4) In this process, the casting temperature is controlled at around 1540℃.

[0075] The preparation method of Example 3 is the same as that of Example 1, except that:

[0076] (5) Homogenize at 1180℃ for 2 hours.

[0077] The preparation method of Example 4 is the same as that of Example 1, except that:

[0078] In (5), the initial forging temperature and the final forging temperature are set to 1100℃ and 950℃, respectively.

[0079] The preparation method of Example 5 is the same as that of Example 1, except that:

[0080] (5) The diameter after forging is 55cm.

[0081] The preparation method of Example 6 is the same as that of Example 1, except that:

[0082] (6) The heat treatment process includes water quenching after holding at 830℃ for 2 hours, followed by annealing at 325℃ for 2 hours, and finally air cooling.

[0083] The inventors conducted extensive research during the research process, and some suboptimal solutions are presented here as comparative examples.

[0084] Comparative Example 1

[0085] The comparative example 1 provides an invar alloy and a preparation method thereof, the components of which are shown in Table 1 above, in the preparation method, only the manganese material is added and the holding step in the micro-alloying stage of step (3) is retained, and no zirconium material and cerium material are added, and the remaining process steps are consistent with those of Example 1, which will not be repeated here.

[0086] Comparative Example 2

[0087] The comparative example 1 provides an invar alloy and a preparation method thereof, the components of which are shown in Table 1 above, in the preparation method, only the manganese material is added and the holding step in the micro-alloying stage of step (3) is retained, and no zirconium material and cerium material are added, and the remaining process steps are consistent with those of Example 1, which will not be repeated here.

[0088] Comparative Example 3

[0089] The comparative example 1 provides an invar alloy and a preparation method thereof, the components of which are shown in Table 1 above, in the preparation method, only the manganese material is added and the holding step in the micro-alloying stage of step (3) is retained, and no zirconium material and cerium material are added, and the remaining process steps are consistent with those of Example 1, which will not be repeated here.

[0090] Comparative Example 4

[0091] The comparative example 1 provides an invar alloy and a preparation method thereof, the components of which are shown in Table 1 above, in the preparation method, only the manganese material is added and the holding step in the micro-alloying stage of step (3) is retained, and no zirconium material and cerium material are added, and the remaining process steps are consistent with those of Example 1, which will not be repeated here.

[0092] Comparative Example 5

[0093] The comparative example 1 provides an invar alloy and a preparation method thereof, the components of which are shown in Table 1 above, in the preparation method, only the manganese material is added and the holding step in the micro-alloying stage of step (3) is retained, and no zirconium material and cerium material are added, and the remaining process steps are consistent with those of Example 1, which will not be repeated here.

[0094] Figure 1 The thermal expansion coefficient curve of the alloy prepared in Example 1 and Comparative Example 1 of the application;

[0095] Figure 2 The room temperature tensile stress-strain curve of the alloy prepared in Example 1 and Comparative Example 1 of the application;

[0096] Figure 3 The potentiodynamic polarization curve of the alloy prepared in Example 1 and Comparative Example 1 of the application.

[0097] Table 1 Main chemical components, wt%

[0098] Ni Mn Zr Ce C O S Example 1 36.01 0.40 0.20 0.020 0.0033 0.0009 0.0004 Example 2 36.98 0.21 0.15 0.010 0.0030 0.0014 0.0008 Example 3 35.51 0.32 0.30 0.015 0.0021 0.0010 0.0006 Example 4 36.49 0.59 0.25 0.020 0.0028 0.0006 0.0004 Example 5 35.02 0.51 0.10 0.025 0.0022 0.0009 0.0004 Example 6 36.00 0.45 0.20 0.030 0.0023 0.0006 0.0004 Comparative Example 1 35.97 0.40 - - 0.0054 0.0108 0.0032 Comparative Example 2 36.03 0.40 - 0.017 0.0024 0.0014 0.0011 Comparative Example 3 36.00 0.40 0.22 - 0.0034 0.0034 0.0031 Comparative Example 4 35.89 0.40 0.49 0.032 0.0032 0.0008 0.0007 Comparative Example 5 36.01 0.40 0.22 0.015 0.0031 0.0014 0.0008

[0099] Table 2 Microstructure of the alloy

[0100]

[0101]

[0102] Potentiodynamic polarization and electrochemical impedance spectroscopy tests were performed using an electrochemical workstation (model Gamry Reference 600). The electrolyte used for the electrochemical tests was a 3.5% (mass fraction) sodium chloride solution, and the size of the test sample was 10 mm x 10 mm x 5 mm (length x width x height), and the test temperature was controlled at 25 ± 1 °C by a super constant temperature water bath. All electrochemical experiments used a conventional three-electrode system, which included a saturated calomel electrode (SCE) as a reference electrode, a Pt sheet electrode as a counter electrode, and a test sample as a working electrode. The back of the sample was connected to a copper wire by soldering and was embedded in epoxy resin, leaving a 10 mm x 10 mm working surface. Before testing, the sample was first polished with 240-5000 mesh SiC sandpaper in stages. Then, the sample was polished with 2.5, 1.5 and 0.5 μm diamond polishing paste. Finally, the sample was finally polished with a 50 nm silica suspension. After completing the polishing step, the sample was cleaned with distilled water and ethanol. When performing electrochemical measurements, first, a cathodic polarization at -1.2 V (vs SCE) for 300 s was performed to remove the oxide film formed in the air. Then, an open circuit voltage was applied to the test sample for 1200 s to allow the electrolytic cell to enter a steady state. The potentiodynamic polarization test was started at -1.0 V (vs SCE) and scanned in the positive direction at a scan rate of 5 mV-s -1 -2 When the current density reached 10 mA-cm

[0103] Table 3 Test results of the alloy

[0104]

[0105]

[0106] The above description is only the preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any changes or substitutions within the technical range disclosed by the present application can be easily thought of by those skilled in the art, which should be covered within the protection scope of the present application.​

Claims

1. An Invar alloy, characterized in that, The composition of the Invar alloy, by mass percentage, includes: Ni: 35.0%–37.0%, C: ≤0.03%, Si: ≤0.30%, Mn: 0.20%–0.60%, Zr: 0.10%–0.30%, Ce: 0.01%–0.03%, with the balance being Fe and unavoidable impurities.

2. The Invar alloy according to claim 1, characterized in that, The microstructure of the Invar alloy includes equiaxed single-phase austenite and second-phase particles dispersed in the austenite matrix, the second-phase particles mainly comprising Ni7Zr2 intermetallic compounds.

3. The Invar alloy according to claim 2, characterized in that, The second phase particles also include Ce2O2S.

4. The Invar alloy according to any one of claims 1 to 3, characterized in that, The Invar alloy has a coefficient of thermal expansion α ≤ 0.61 × 10⁻⁶ within the temperature range of 20℃ to 100℃. -6 / ℃, yield strength above 400MPa, tensile strength above 500MPa.

5. A method for preparing the Invar alloy according to any one of claims 1 to 4, characterized in that, Includes the following steps: Step 1: Weigh out the iron, nickel, manganese, zirconium and cerium materials according to their mass percentages. Put the iron and nickel materials into the crucible, and put the manganese, zirconium and cerium materials into the silos separately. Step 2: Smelt the iron and nickel materials. After the iron and nickel materials are melted and clear, add the manganese material to the molten pool and keep it warm; then add the cerium material and keep it warm; finally add the zirconium material and keep it warm. Step 3: Casting to obtain an ingot; Step 4: Homogenize the ingot and then forge it to obtain a forging billet; Step 5: Perform solution treatment and aging treatment on the forging billet to obtain Invar alloy.

6. The preparation method according to claim 5, characterized in that, In step 2, manganese material is added to the molten pool and kept at a constant temperature for 5-10 minutes, and then cerium material is added and kept at a constant temperature for 5-10 minutes.

7. The preparation method according to claim 5, characterized in that, In step 3, the casting temperature is 1500–1550℃.

8. The preparation method according to claim 5, characterized in that, In step 4, the homogenization process includes heating the ingot to 1180-1210°C and holding it at that temperature for 1-2 hours.

9. The preparation method according to claim 5, characterized in that, In step 4, the initial forging temperature is controlled at 1050-1200℃ and the final forging temperature is controlled at 850-950℃.

10. The preparation method according to any one of claims 5 to 9, characterized in that, In step 5, the solution treatment and aging treatment include: water quenching after holding at 830-850℃ for 1-2 hours, followed by air cooling after holding at 305-325℃ for 1-2 hours.

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