Nickel-iron-based heat-resistant alloy and preparation method thereof

Abstract

The invention discloses a nickel-iron-based heat-resistant alloy and a preparation method thereof, belongs to the technical field of heat-resistant alloys, and solves the problems that in the prior art, the service temperature of an existing heat-resistant alloy is low, and the service life is short when the existing heat-resistant alloy is used at the service temperature of 850 DEG C. The nickel-iron-based heat-resistant alloy is prepared from the following components in percentage by mass: 0.025 percent to 0.075 percent of C, 52 percent to 58 percent of Ni, 17.0 percent to 20.0 percent of Cr, 7.0 percent to 8.5 percent of Mo, 2.0 percent to 3.5 percent of W, 0.4 percent to 1.0 percent of Mn, 0.05 percent to 0.2 percent of Al, 0.04 percent to 0.15 percent of Zr, 0.005 percent to 0.03 percent of N, 0.002 percent to 0.006 percent of B, 0.01 percent to 0.03 percent of Ce and the balance of iron and inevitable impurity elements. The nickel-iron-based heat-resistant alloy is excellent in comprehensive performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of heat-resistant alloys, in particular to a nickel-iron-based heat-resistant alloy and a preparation method thereof. BACKGROUND

[0002] With the increasing demand for energy in China, nuclear energy, as one of the clean energies, plays a huge role in the energy system facing the goal of "carbon peak and carbon neutralization". The fourth generation nuclear energy system represents the development trend of advanced nuclear energy systems, among which the high temperature gas cooled reactor has the potential to achieve commercial application. At present, the temperature of the helium outlet of the high temperature gas cooled reactor can reach 750~950℃, and the corresponding metal core support structure, steam generator and intermediate heat exchanger in the reactor require structural materials to meet the mechanical properties under high temperature. Therefore, nickel-based and nickel-iron-based heat-resistant alloys with good thermal strength and thermal stability have become candidate materials for high temperature components of gas cooled reactors.

[0003] Although there are some heat-resistant alloys at home and abroad, the service temperature of these heat-resistant alloys is mostly below 800℃ and the service life is relatively short, and the requirement for the high temperature structure and performance stability of the material is relatively low. Therefore, it is urgent to develop a heat-resistant alloy material that can be applied to a service temperature of 850℃ and has good processability. SUMMARY

[0004] In view of the above, the present application aims to provide a nickel-iron-based heat-resistant alloy and a preparation method thereof, which at least solve one of the following technical problems: the service temperature of the existing heat-resistant alloy is low, the long-term life at a service temperature of 850℃ is short, or the welding performance is poor.

[0005] The purpose of the present application is mainly realized by the following technical scheme: The present application provides a nickel-iron-based heat-resistant alloy, the components of the nickel-iron-based heat-resistant alloy include, in terms of mass percentage: C: 0.025%~0.075%, Ni: 52%~58%, Cr: 17.0%~20.0%, Mo: 7.0%~8.5%, W: 2.0%~3.5%, Mn: 0.4%~1.0%, Al: 0.05%~0.2%, Zr: 0.04%~0.15%, N: 0.005%~0.03%, B: 0.002%~0.006%, Ce: 0.01%~0.03%, and the balance is iron and unavoidable impurity elements.

[0006] Further, in the nickel-iron-based heat-resistant alloy, the content of Cr, Mo and W satisfies the relationship formula: 155≤100Cr×(100Mo+50W)≤195, wherein Cr, Mo and W respectively represent the mass percentage of elements Cr, Mo and W in the nickel-iron-based heat-resistant alloy.

[0007] Further, the content of Zr and N in the nickel-iron-based heat-resistant alloy satisfies the relationship: -0.0009+163N < 100Zr < -0.00122+720N, wherein Zr and N respectively represent the mass percentage of elements Zr and N in the nickel-iron-based heat-resistant alloy.

[0008] Further, the components of the nickel-iron-based heat-resistant alloy include, in mass percentage: C: 0.035%~0.06%, Ni: 54%~57%, Cr: 17.2%~20.0%, Mo: 7.3%~8.5%, W: 2.5%~3.5%, Mn: 0.6%~0.9%, Al: 0.08%~0.15%, Zr: 0.04%~0.15%, N: 0.008%~0.025%, B: 0.003%~0.005%, Ce: 0.011%~0.026%, and the balance being iron and inevitable impurity elements.

[0009] Further, the impurity elements in the components of the nickel-iron-based heat-resistant alloy include, in mass percentage: Si: ≤0.30%, Co ≤0.2%, Cu: ≤0.07%, S ≤0.005%, P ≤0.008%, O ≤10ppm, H ≤1ppm, Pb ≤0.001%, Sb ≤0.0025%, Sn ≤0.0012%, Bi ≤0.001%, and As ≤0.002%.

[0010] Further, the microstructure of the nickel-iron-based heat-resistant alloy mainly includes austenitic equiaxed grains and dispersedly distributed precipitates; wherein the precipitates mainly include M6C and Zr(C,N) carbides.

[0011] Further, in the microstructure of the nickel-iron-based heat-resistant alloy, the mass percentage of M6C is 0.20%~0.35%, and the mass percentage of Zr(C,N) is 0.05%~0.15%.

[0012] The application also provides a preparation method of the above nickel-iron-based heat-resistant alloy, including the following steps: Step 1, smelting to obtain an ingot; Step 2, homogenizing treatment of the ingot; Step 3, forging the ingot to obtain a forged blank; Step 4, solid solution treatment of the forged blank to obtain the nickel-iron-based heat-resistant alloy.

[0013] Further, in Step 2, the homogenizing treatment includes: heating the ingot to 1150~1180℃, holding for 24~50h after the ingot is heated through, and then furnace cooling to room temperature.

[0014] Further, in Step 3, the open forging temperature is controlled to be 1100~1150℃, and the finish forging temperature is controlled to be 1000~1010℃.

[0015] Compared with the prior art, the present application can at least achieve one of the following beneficial effects: The nickel-iron-based heat-resistant alloy of the present application, by controlling the content of Ni and Fe elements to meet a certain ratio, with Ni element as the main base element and Fe element as the auxiliary base element, on the one hand, stabilizes the thermal stability of the austenitic base, improves the ability of the alloy to solid-solve Cr, W, Mo and other elements, and avoids the precipitation of harmful phases. On the other hand, the good oxidation and corrosion resistance of Ni element at high temperature makes it possible for high-temperature components for nuclear power to serve at high temperature for a long time; in addition, the nickel-iron-based alloy makes the raw material cost lower than that of nickel-based alloy; by controlling the content of Cr, W and Mo elements, while improving the solid solution strengthening effect, the precipitation of harmful sigma phase in the nickel-iron-based alloy is significantly inhibited, ensuring the microstructure thermal stability of the alloy under the service condition of the highest 950℃ gas cooled reactor. The appropriate amount of Cr element not only ensures the oxidation and corrosion resistance of the alloy, but also avoids the precipitation of continuous carbides at the grain boundary, which is beneficial to the impact toughness. Moderate Mo and W elements not only improve the strength, but also avoid the problem of significant segregation of the alloy during the solidification stage and the large deformation resistance during the forging process, so that the alloy has good thermal processing performance and low welding crack sensitivity after homogenization; the appropriate amount of Zr element and trace amounts of B and N elements not only improve the morphology of the grain boundary carbides, but also improve the grain boundary bonding strength, purify the grain boundary, improve the creep endurance and high temperature plasticity, and avoid the precipitation of low melting point phases and their adverse effects on the process performance such as thermal processing and welding; Co element is identified as an impurity element and the Co content in the furnace charge is limited to 0.2%, which effectively avoids the problem of radioactive pollution caused by Co element when the nickel-iron-based alloy high-temperature component is served in the irradiation environment; by controlling the content of each element in the alloy within a suitable range, the alloy has excellent comprehensive performance and low raw material cost.

[0016] The preparation method of the present application precisely controls each process step and process parameter, ensuring that the microstructure of the nickel-iron-based heat-resistant alloy mainly includes austenitic equiaxed grains and dispersedly distributed precipitates; the precipitates mainly include M6C and Zr(C,N) carbides, M 23 C6 carbides are basically dissolved in the matrix after solid solution treatment, and M6C and Zr(C,N) carbides are in the form of granules or blocks and are randomly and uniformly distributed in the matrix; the content of M6C is about 0.20%~0.35% (mass fraction); the content of Zr(C,N) is about 0.05%~0.15% (mass fraction), and the grain size is controlled at 3.5~5 levels. Further, the excellent performance of the nickel-iron-based heat-resistant alloy is ensured.

[0017] The nickel-iron-based heat-resistant alloy of the present application has excellent comprehensive performance, for example: room temperature mechanical properties: when the test temperature is 20℃, the tensile strength: R m≥ 740 MPa, for example 740 ~ 770 MPa; yield strength: R p0.2 ≥ 290 MPa, for example 295 ~ 320 MPa; elongation: A ≥ 48%, for example 49.5% ~ 55%; reduction of area: Z ≥ 65%, for example 66% ~ 78%; impact absorbed energy: KV2 ≥ 230 J, for example 235 ~ 270 J; high temperature mechanical property: when the test temperature is 850℃, tensile strength: R m ≥ 270 MPa, for example 280 ~ 320 MPa; yield strength: R p0.2 ≥ 185 MPa, for example 190 ~ 210 MPa; elongation: A ≥ 50%, for example 85% ~ 96%; reduction of area: Z ≥ 80%, for example 82% ~ 92%; 850℃ / 50MPa endurance life: δ ≥ 890h, for example 899 ~ 950h. The performance of the nickel-iron-based heat-resistant alloy of the present application meets the performance requirements of high-temperature components for high-temperature gas cooled reactors at 850℃, and has excellent hot working performance and excellent welding performance.

[0018] Other features and advantages of the present application will be set forth in the following description, 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 present application. The objects and other advantages of the present 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

[0019] 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:

[0020] Figure 1 is the relationship between the content of Zr and N and the formation of Ni5Zr phase and CrN phase; Figure 2a is the true stress-true strain curve of the nickel-iron-based heat-resistant alloy of the present application; Figure 2b is the true stress-true strain curve of the nickel-iron-based heat-resistant alloy of the present application; Figure 2c is the true stress-true strain curve of the nickel-iron-based heat-resistant alloy of the present application; Figure 3 is the forged state structure of the nickel-iron-based heat-resistant alloy obtained by the preparation method in the present application; Figure 4 is the solid solution state structure of the nickel-iron-based heat-resistant alloy obtained by the preparation method in the present application; Figure 5a is the SEM image of the nickel-iron-based heat-resistant alloy prepared in Example 1; Figure 5b is the SEM image of the nickel-iron-based heat-resistant alloy prepared in Example 1; Figure 6 SEM image of the nickel-iron-based heat-resistant alloy prepared for Comparative Example 5; Figure 7 SEM image of the nickel-iron-based heat-resistant alloy prepared for Comparative Example 7; Figure 8 SEM image of the nickel-iron-based heat-resistant alloy prepared for Comparative Example 13. DETAILED DESCRIPTION

[0021] The preferred embodiments of the present application will be described in detail below with reference to the drawings, in which the drawings constitute a part of this application and illustrate embodiments of the present application together with the principles of the present application.

[0022] The present application provides a nickel-iron-based heat-resistant alloy, the components of the nickel-iron-based heat-resistant alloy include, in terms of mass percentage: C: 0.025%~0.075%, Ni: 52%~58%, Cr: 17.0%~20.0%, Mo: 7.0%~8.5%, W: 2.0%~3.5%, Mn: 0.4%~1.0%, Al: 0.05%~0.2%, Zr: 0.04%~0.15%, N: 0.005%~0.03%, B: 0.002%~0.006%, Ce: 0.01%~0.03%, and the balance being iron and inevitable impurity elements.

[0023] The effects of the components contained in the present application and the selection of the amount are described in detail below. Carbon (C): C element is an important carbide forming element in the alloy, the addition of C can promote the precipitation of M6C, M 23 C6, MC and other carbides, which are distributed in the grain boundary and the grain, so as to pin the dislocation and hinder the movement of the grain boundary, thereby improving the creep endurance of the alloy. At the same time, C element can fill the area between the grain boundary and the dendrite as an interstitial element, slow down the diffusion, and strengthen the grain boundary. The addition of C element can also purify the alloy, improve the purity of the alloy, reduce the oxides in the alloy, and improve the castability of the alloy; in addition, the increase of C content also increases the amount of carbide precipitation, especially the M6C carbide, which strengthens the pinning effect of the grain boundary, and significantly improves the strength and hardness. However, when the C content exceeds a certain value, the organization will appear aggregated carbide and continuous carbide along the grain, which on the one hand increases the solid solution difficulty of alloy elements, weakens the solid solution strengthening effect of the alloy, and on the other hand increases the sensitivity of the alloy to the liquid cracking and stress relaxation cracking in the heat affected zone (HAZ), and the plasticity and toughness decrease significantly, which is also very unfavorable to the welding performance. Therefore, the mass percentage range of C in the present application is controlled to be 0.025%~0.075%.

[0024] Nickel (Ni): Ni element as one of the base elements of the alloy, plays an important role in the thermal strength, thermal stability, oxidation corrosion resistance of the alloy. Ni element has a face-centered cubic structure, and the austenitic matrix with Ni as the base is called γ, without allotropy transformation, and has higher thermal stability than the body-centered cubic Fe element and the close-packed hexagonal Co element. The Ni-based alloy can solid solution more alloying elements without generating harmful phases, M 23 C6 carbide after heat treatment are dissolved into the matrix to ensure solid solution strength, while the Fe or Fe-Cr matrix can only solid solution less alloying elements, and has the tendency to precipitate various harmful phases. This feature provides the possibility to improve the microstructure and performance stability of nickel. On the other hand, the face-centered cubic austenite has higher high-temperature strength than the body-centered cubic ferrite, because the atomic diffusion ability of face-centered cubic austenite is small, i.e. the self-diffusion activation energy is higher. Therefore, in the alloy design, it is necessary to ensure that the nickel-iron-based alloy is in a face-centered cubic structure at any temperature. In addition to thermal strength and microstructure stability, Ni element has good oxidation and corrosion resistance in the range of 650-1000℃. In summary, the mass percentage of Ni in the present application is controlled in the range of 52%-58%.

[0025] Chromium (Cr): Cr element not only can ensure the oxidation resistance and hot corrosion resistance of the material, but also has higher solid solution strengthening effect, and can form M 23 C6 and M6C carbides with C element in the material, effectively prevent grain boundary sliding and migration, thereby improving the high temperature creep strength of the material. If the content of Cr is less than 16%, the alloy not only has insufficient oxidation resistance, but also affects its solid solution strengthening effect, and if Cr exceeds 24%, it will promote the precipitation of harmful TCP phase, increase the cracking tendency, and reduce the microstructure stability. Considering comprehensively, too high Cr element in the nickel-iron-based alloy of the present application will significantly promote the precipitation of brittle phases μ phase and σ phase, and may also form grain boundary continuous carbide, which affects the high temperature mechanical properties. Therefore, the mass percentage of Cr in the present application is controlled in the range of 17.0%-20.0%.

[0026] Tungsten (W) and Molybdenum (Mo): W and Mo elements are the main solid solution strengthening elements in the nickel-iron-based alloy, which contribute greatly to the solid solution strengthening of the alloy. W and Mo both belong to large-size elements, and the atomic radius difference between them and Ni, Fe is large. The solid solution of the two will significantly increase the degree of lattice distortion and form a large long-range stress field, thereby improving the strength of the alloy. Both of them can also significantly reduce the stacking fault energy and diffusion coefficient of the alloy, increase the diffusion activation energy, slow down the high-temperature diffusion speed of Al, Ti and Cr, and strengthen the atomic binding force of the solid solution. Among them, compared with Mo element, the atomic radius and melting point of W element are larger, and the atomic mass is about twice that of Mo atomic mass. The solid solution strength contribution value of W element in nickel-based and nickel-iron-based alloy is about twice that of equal amount of Mo element. In addition, W and Mo elements are also the forming elements of carbides M6C and M 23 C6. Due to the low diffusion coefficient of W and Mo, the alloy is prone to segregation during solidification, resulting in uneven microstructure of the alloy ingot and the welding performance of the alloy also decreases, which is not conducive to the manufacture of large-size forgings. In addition, it is found through in-depth research that in the nickel-iron-based alloy, a higher content of W will promote the precipitation of μ phase and the increase of M6C dissolution temperature, and the increase of Mo element will increase the content of σ phase and M6C phase. Therefore, in the design of W and Mo elements, the strength is ensured as much as possible, and the stability of precipitated phase, welding performance and hot working performance of the alloy are also considered. Considering comprehensively, the mass percentage of Mo is controlled to be 7.0% to 8.5%, and the mass percentage of W is controlled to be 2.0% to 3.5%.

[0027] Manganese (Mn): In the system of the present application, the addition of Mn element not only can play a role of deoxidization and desulfurization, but also can improve the oxidation resistance and welding performance of the alloy. First, Mn element can act as a refining agent to generate MnS (melting point 1620℃) by chemical reaction with S element, thereby reducing the harmful effect of S, which is related to the improvement of the hot workability and high-temperature corrosion resistance of the nickel-based alloy; in addition, Mn is easy to combine with O element to inhibit the formation of low-melting silicates and reduce the tendency of crystallization cracking; secondly, a small amount of Mn element is beneficial to improve the adhesion and compactness of the oxidation film of the alloy, and form a protective film of MnCr2O4 on the surface of the alloy to prevent further oxidation of the alloy, which is beneficial to the high-temperature comprehensive performance; in terms of welding performance, Mn can refine the grain of the weld and improve the hot cracking resistance of the nickel-based alloy and reduce its sensitivity to ductile crack. When the content of Mn is high, it will be segregated at the grain boundary, weaken the grain boundary bonding force, reduce the endurance strength, and also may form a large amount of Mn-oxide, which is easy to cause hot cracking and is not conducive to the hot working performance. Therefore, in the present application, the content of Mn element is controlled to be 0.4% to 1.0%.

[0028] Aluminum (Al): Adding trace amounts of Al is the most effective measure to improve the cyclic oxidation resistance of nickel-iron-based alloys. Al can promote the formation of a dense and continuous (Cr,Al)₂O₃ oxide layer, playing an excellent role in hindering diffusion and improving the alloy's oxidation resistance. Simultaneously, the bonding strength between the matrix and the oxide layer, as well as the plasticity of the oxide scale, are improved. Studies have shown that γ' phase precipitates when the Al content reaches 1 wt.% without Ti. The formation of the γ' phase is undesirable in solid-solution nickel-iron-based alloys; therefore, adding trace amounts of Al improves the surface stability of the alloy. Thus, in this invention, the Al content is controlled within the range of 0.05% to 0.2%.

[0029] Zirconium (Zr): As a grain boundary strengthening element, Zr accumulates at grain boundaries and boride / matrix interfaces, altering the morphology of the primary MC phase and the second phase at grain boundaries. This facilitates spheroidization of the second phase morphology, improving grain boundary strength. Furthermore, Zr combines with sulfur to form sulfides, purifying grain boundaries and reducing the sulfur content in the alloy. However, the resulting ZrS is a low-melting-point phase, negatively impacting hot working and weldability; therefore, the amount of Zr added should be carefully controlled. Additionally, Zr can increase carbide linear density, thereby enhancing the strengthening effect, refining grain and carbide sizes, and promoting the precipitation of granular carbides at grain boundaries. Excessive Zr addition easily leads to the precipitation of low-melting-point intermetallic compounds such as Ni5Zr and NiZr, severely impairing alloy properties. Therefore, in this invention, the Zr content is controlled within the range of 0.04% to 0.15%.

[0030] Nitrogen (N): In the nickel-iron-based superalloy of the present invention, an appropriate increase in nitrogen helps to form discrete carbides, thereby reducing grain boundary cracking and improving alloy plasticity. Furthermore, during alloy solidification, trace amounts of nitrogen combine with Zr to form the primary phase ZrN, which can be M6C or M... 23 The precipitation of C6 carbides provides heterogeneous nucleation sites, which is beneficial for improving carbide morphology and refining size. When the N content is too high, aggregated nitrides or continuous precipitation of CrN phase along grain boundaries are easily formed in the alloy, which is detrimental to the alloy's plasticity and toughness. Therefore, the N content must be carefully controlled. Thus, in this invention, the N content is controlled within the range of 0.005% to 0.03%.

[0031] Boron (B): B has the most significant impact on the durability and creep properties of heat-resistant alloys. Its solubility in the matrix phase is extremely low, and it is typically used as an interstitial element to fill grain boundaries and dendrite regions, hindering grain boundary atomic diffusion, increasing bonding strength, and reinforcing grain boundaries. Appropriate amounts of B are beneficial for improving the high-temperature strength of nickel-iron-based alloys, but when the B content exceeds 0.010%, a large amount of borides such as M3B2 may be formed, which is detrimental to the high-temperature performance of the alloy. Simultaneously, the low-melting-point borides also increase the tendency to crack. The low-melting-point phase containing B generated during welding solidification can form grain boundary liquidation cracks, severely affecting the weldability of heat-resistant alloys. Therefore, in this invention, the B content is controlled within the range of 0.002% to 0.006%.

[0032] Cerium (Ce): A rare earth element, Ce is highly chemically reactive, exhibiting a strong affinity for both oxygen (O) and sulfur (S). During smelting, it plays a significant role in removing O and S gases and purifying grain boundaries. Ce can reduce the oxidation rate of alloys at different temperatures, minimizing oxidation weight gain and improving hot working properties. Rare earth elements can increase the adhesion between the base metal and oxides, facilitating the formation of Cr2O3 on the material surface and indirectly improving the oxidation and corrosion resistance of nickel-iron alloys. Considering the high reactivity of Ce and its susceptibility to burn-off during smelting, precise yield and uniform distribution control for large ingots are challenging. Therefore, in this invention, the Ce content is controlled within the range of 0.01% to 0.03%.

[0033] Silicon (Si): Si is typically a harmful impurity in nickel-iron based alloys. It tends to segregate at grain boundaries, altering the bonding state of atoms in these regions, reducing grain boundary cohesion, increasing cracking tendency, and leading to decreased high-temperature creep strength and durability. Si can also widen the solidification-liquid temperature range and reduce material plasticity. High Si content promotes the precipitation of lamellar σ phases at grain boundaries and within grains, creating pathways for crack initiation and propagation. Studies have shown that Si content below 0.5% has little effect on the mechanical properties of heat-resistant alloys, while levels above 0.7% lead to a sharp deterioration in performance. Therefore, the alloy of this invention does not contain Si, controlling the Si content to below 0.3%.

[0034] Cobalt (Co): Since the isotope Co-60 is a high-energy gamma-ray emitter, high Co content is detrimental to its application in radiation environments. Therefore, the high Co content of high-temperature components in gas-cooled reactors operating under irradiation conditions may cause potential radioactive contamination problems. Therefore, the alloy of this invention does not contain Co, limiting the Co content in the alloy to less than 0.2%.

[0035] Specifically, the aforementioned nickel-iron-based heat-resistant alloys also contain unavoidable impurity elements and gaseous elements, such as residues generated during the manufacturing process from raw materials, slag, and refractory materials, including Si, Co, Cu, S, P, O, H, Pb, Sb, Sn, Bi, and As. These elements require strict control. By mass percentage, the controlled impurity content in the aforementioned nickel-iron-based heat-resistant alloys is as follows: Si: ≤0.30%, Co≤0.2%, Cu: ≤0.07%, S≤0.005%, P≤0.008%, O≤10ppm, H≤1ppm, Pb≤0.001%, Sb≤0.0025%, Sn≤0.0012%, Bi≤0.001%, and As≤0.002%.

[0036] Specifically, in the nickel-iron-based heat-resistant alloy of the present invention, the contents of Cr, Mo and W satisfy the following relationship: 155≤100Cr×(100Mo+50W)≤195, where Cr, Mo and W refer to the mass percentages of the elements Cr, Mo and W in the nickel-iron-based heat-resistant alloy, respectively.

[0037] In this invention, the solid solubility of Cr, Mo, and W elements in nickel-iron-based alloys exhibits a certain relationship, significantly influencing the precipitation of carbides and harmful phases. Mo is the main forming element of the σ phase and M6C carbides, while W is the main forming element of the harmful μ phase. Increased Cr content can promote the precipitation of the σ phase and M6C carbides. 23 The precipitation of C6 and M6C carbides. In nickel-iron-based alloys, the σ and μ phases are brittle phases, serving as pathways for crack initiation and propagation, and also weakening the solid solution strengthening effect. Excessive Cr, Mo, and W elements lead to the precipitation of harmful phases and aggregated carbides, which are detrimental to hot working properties and oxidation corrosion resistance, and also easily cause welding solidification cracks. On the other hand, low Cr, Mo, and W elements make it difficult to meet the alloy's requirements for high-temperature strength and creep resistance. Taking all factors into consideration, this invention controls 155 ≤ 100Cr × (100Mo + 50W) ≤ 195, ensuring high-temperature strength while also taking into account the alloy's phase stability, oxidation corrosion resistance, weldability, and hot working properties.

[0038] In this invention, the contents of Zr and N satisfy the following relationship: -0.0009+163N<100Zr<-0.00122+720N, where Zr and N refer to the mass percentages of Zr and N in the nickel-iron-based heat-resistant alloy, respectively.

[0039] In this invention, different Zr and N element ratios can precipitate different types of intermetallic compounds. When the Zr content is 0.04% and the N content is ≤0.00582%, Ni5Zr phase and Zr(C,N) carbonitrides precipitate in the alloy microstructure. Ni5Zr phase is a low-melting-point phase with a melting temperature of 1140~1170℃, which is detrimental to the alloy's homogenization, hot workability, weldability, and creep rupture performance. When the N content is too high, the alloy microstructure precipitates a continuous CrN phase along the grain boundaries at 850℃. This phase is a brittle phase with a close-packed hexagonal structure, and its continuous distribution and coarsening behavior severely impair the alloy's ductility and toughness. Therefore, by optimizing the Zr and N element contents, with the goals of "no precipitation of Ni5Zr phase" and "no precipitation of CrN phase at 850℃," a relationship between the Zr and N content ratio and the formation of Ni5Zr and CrN phases is established, as follows: Figure 1 As shown. Taking all factors into consideration, when the Zr content is controlled at 0.04~0.15% and the N content at 0.005%~0.03%, and the above relationship is satisfied (-0.0009+163N<100Zr<-0.00122+720N), the microstructure and properties of the alloy reach their optimal levels. This fully utilizes the strengthening effects of Zr and N elements while avoiding the formation of harmful phases Ni5Zr and CrN, further improving the alloy's processing performance, ductility, toughness, and high-temperature stability.

[0040] Specifically, in order to further improve the overall performance of the above-mentioned nickel-iron-based heat-resistant alloy, the composition of the above-mentioned nickel-iron-based heat-resistant alloy, by mass percentage, includes: C: 0.035%~0.06%, Ni: 54%~57%, Cr: 17.2%~20.0%, Mo: 7.3%~8.5%, W: 2.5%~3.5%, Mn: 0.6%~0.9%, Al: 0.08%~0.15%, Zr: 0.04%~0.15%, N: 0.008%~0.025%, B: 0.003%~0.005%, Ce: 0.011%~0.026%, with the balance being iron and unavoidable impurity elements.

[0041] On the other hand, the present invention also provides a method for preparing the above-mentioned nickel-iron-based heat-resistant alloy, comprising the following steps: Step 1: Smelting to obtain ingots; Step 2: Homogenize the ingot; Step 3: Forge the ingot to obtain a forging billet; Step 4: Perform solution treatment on the forging billet to obtain a nickel-iron-based heat-resistant alloy.

[0042] Specifically, in step 1 above, the smelting adopts a two-stage process of vacuum induction melting + vacuum consumable remelting or a three-stage process of vacuum induction melting + protective atmosphere electroslag remelting + vacuum consumable remelting to produce consumable ingots with uniform composition, low segregation, and good surface quality, while ensuring the purity of the alloy.

[0043] Specifically, in step 2 above, the homogenization process includes: heating the ingot to 1150~1180℃, holding the ingot at that temperature for 24~50h after it is fully heated, and then cooling it to room temperature in the furnace.

[0044] It should be noted that the purpose of homogenization treatment is to eliminate compositional and microstructure segregation. Excessively high holding temperatures can lead to the melting of low-melting-point phases at grain boundaries (overheating), causing irreversible grain boundary weakening and embrittlement. Excessively long holding times can cause abnormal grain coarsening, impairing the material's toughness and strength, and potentially causing the loss of beneficial strengthening elements. Conversely, holding temperatures that are too low or holding times that are too short cannot adequately eliminate as-cast dendritic segregation; residual brittle phases and inhomogeneous structures will significantly reduce the alloy's hot working plasticity, long-term service stability, and overall mechanical properties. Therefore, the holding temperature should be controlled at 1150~1180℃, for example, 1150℃, 1160℃, 1170℃, 1180℃; and the ingot should be held for 24~50 hours after through-heating, for example, 24 hours, 30 hours, 35 hours, 40 hours, 45 hours, 50 hours.

[0045] Specifically, in step 3 above, the true stress-true strain curve of the alloy of this invention was obtained after in-depth research, as shown in Figure 2. When the deformation temperature is below 1050℃, the deformation resistance is too high, the recrystallization degree is low, and the uniformity of the deformed structure is poor. When the deformation temperature is above 1150℃, the deformation resistance is low, and the structure is relatively coarse. Taking all factors into consideration, the initial forging temperature is controlled at 1100~1150℃, for example, 1100℃, 1110℃, 1120℃, 1130℃, 1140℃, or 1150℃; the final forging temperature is controlled at 1000~1010℃, for example, 1000℃, 1005℃, or 1010℃. Controlling the above process can ensure that the deformation difficulty is low and that a forged structure with uniform carbide distribution, high recrystallization degree, and uniform and fine grains can be obtained.

[0046] It should be noted that the temperature control of the present invention can allow an error of ±5℃.

[0047] Specifically, in step 3 above, in order to avoid cracking or uneven structure caused by excessive temperature difference in the forging billet during the forging process, a soft sheathing process is adopted. The sheathing material is aluminum silicate ceramic fiber blanket with a thickness of 12~15mm, which can effectively reduce heat loss and ensure the deformation amount per forging and the uniformity of the structure of the forging.

[0048] Specifically, in step 4 above, the solution treatment includes the following steps: heating the forging billet to 1125~1175℃ and holding it at that temperature for 55~65 minutes, followed by water cooling to room temperature.

[0049] Specifically, in step 4 above, the purpose of solution treatment is to heat the forged alloy to a certain temperature and hold it for a period of time, so that the precipitated carbides can dissolve into the austenite matrix as much as possible, and fully utilize the solid solution strengthening effect of alloying elements such as Cr, W, and Mo. Considering the carbide remelting temperature and grain growth kinetics, the solution treatment parameters include heating the forged billet to 1125~1175℃ and holding it at that temperature, followed by water cooling to room temperature, which can dissolve M... 23 C6 carbides are largely dissolved into the matrix, while a small amount of fine and uniformly distributed Zr(C,N) carbonitrides and M6C carbides remain, such as Figure 4 As shown, after solution treatment, a uniformly distributed solid solution structure with a high degree of solid solution was obtained.

[0050] Specifically, the microstructure of the nickel-iron-based heat-resistant alloy obtained in step 4 above mainly consists of austenitic equiaxed grains and dispersed precipitates; the precipitates mainly include M6C and Zr(C,N) carbides, M... 23 After solution treatment, C6 carbides are largely dissolved into the matrix, while M6C and Zr(C,N) carbides are granular or blocky and randomly and uniformly distributed in the matrix. The mass percentage of M6C is approximately 0.20% to 0.35%; the mass percentage of Zr(C,N) is approximately 0.05% to 0.15%, and the grain size is controlled at grade 3.5 to 5 (e.g., grade 4 to 5).

[0051] Specifically, the nickel-iron-based heat-resistant alloy obtained in step 4 above exhibits excellent comprehensive properties, for example: (1) Mechanical properties at room temperature: Tensile strength at a test temperature of 20℃: R m ≥740MPa, for example 740~770MPa; Yield strength: R p0.2 ≥290MPa, for example 295~320MPa; Elongation: A≥48%, for example 49.5%~55%; Reduction of area: Z≥65%, for example 66%~78%; Impact absorption energy: KV2≥230J, for example 235~270J; (2) High-temperature mechanical properties: Tensile strength at a test temperature of 850℃: R m ≥270MPa, for example 280~320MPa; Yield strength: R p0.2 ≥185MPa, for example 190~210MPa; Elongation: A≥50%, for example 85%~96%; Reduction of area: Z≥80%, for example 82%~92%; Duration at 850℃ / 50MPa: δ≥890h, for example 899~950h.

[0052] The advantages of precise control over the composition and process parameters of the alloy of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0053] Examples 1-5 of the present invention provide a nickel-iron-based heat-resistant alloy and its preparation method. The chemical composition of the alloys in Examples 1-5 is shown in Table 1, and the relationship between the alloying elements is shown in Table 2.

[0054] The preparation method of Example 1 includes: (1) The raw material ratio is determined according to the mass percentage of each element, and the alloy is smelted by vacuum induction melting + vacuum self-consumable remelting process and cast into 200kg alloy ingots, with strict control over the content of impurity elements; (2) The ingot obtained in step 1 is homogenized by holding it at 1150℃ for 27 hours, and then furnace cooled to room temperature; (3) The alloy ingot obtained in step 2 is forged into a forging with a diameter of 18 mm. The initial forging temperature is 1100℃ and the final forging temperature is 1000℃. The soft sleeve process is used during the forging process to ensure the deformation per forging and the uniformity of the microstructure of the forging. (4) The forging obtained in step 3 is subjected to solution treatment. The forging is placed in a high-temperature furnace. When the overall temperature of the forging reaches 1130℃, it is kept at the temperature for 60 minutes. Then it is cooled to room temperature by water to obtain a solution-treated nickel-iron-based heat-resistant alloy.

[0055] The preparation method of Example 2 is generally the same as that of Example 1, except that: (2) In this case, the temperature is kept at 1160℃ for 26 hours; (3) The forging temperature is 1110℃.

[0056] The preparation method of Example 3 is generally the same as that of Example 1, except that: (2) In this case, the temperature is kept at 1170℃ for 25 hours; (3) In this case, the forging temperature is 1120℃; (4) After the overall temperature of the forging bar reaches 1140℃, it is kept at that temperature for 57 minutes.

[0057] The preparation method of Example 4 is generally the same as that of Example 1, except that: (3) The initial forging temperature is 1130℃ and the final forging temperature is 1005℃.

[0058] The preparation method of Example 5 is generally the same as that of Example 1, except that: (4) After the overall temperature of the forging bar reaches 1150℃, it is kept at that temperature for 55 minutes.

[0059] The inventors conducted extensive research during the research process, and some suboptimal solutions are presented here as comparative examples.

[0060] Comparative Examples 1-12 A comparative example provides a nickel-iron-based heat-resistant alloy and its preparation method. The composition is shown in Table 1, and the relationship between the alloying elements is shown in Table 2. The preparation method of the comparative example is the same as that of Example 1, and will not be repeated here.

[0061] Comparative Example 13 This comparative example provides a nickel-iron-based heat-resistant alloy and its preparation method. Its composition is the same as that of Example 1, and the preparation method is generally the same as that of Example 1, except that: (2) In this case, the temperature is kept at 1100℃ for 25 hours; (3) In this case, the forging temperature is 1050℃; (4) After the overall temperature of the forging bar reaches 1155℃, it is kept at that temperature for 58 minutes.

[0062] The microstructure of the nickel-iron-based heat-resistant alloy obtained in this comparative example mainly consists of austenitic grains and precipitates that have not dissolved into the matrix. Although a suitable solution heat treatment process was subsequently employed, the homogenization temperature and forging temperature were too low. On the one hand, this failed to ensure the complete elimination of dendritic segregation of elements such as Cr, W, and Mo in the as-cast microstructure, and on the other hand, it was difficult to obtain a fully recrystallized forged microstructure. After solution heat treatment, the grain size in the microstructure was relatively fine, but the overall microstructure homogeneity was poor, and locally aggregated blocky M6C carbides appeared. Figure 8 SEM image of the nickel-iron-based heat-resistant alloy prepared for Comparative Example 13.

[0063] The alloys prepared in the examples and comparative examples were characterized by metallographic (OM) and scanning electron microscopy (SEM). The samples were mechanically ground and polished using SiC sandpaper and diamond polishing agent, then wiped with cotton soaked in an etching solution (2g CuCl2 + 40 ml HCl + 60 ml ethanol) for 20–25 seconds to reveal the microstructure. The grain size was determined by the cutoff point method. Simultaneously, the precipitated phases in the microstructure were extracted, and their mass fractions were statistically analyzed. The results are shown in Table 3.

[0064] The main performance test results of the examples and comparative examples are shown in Tables 4-5.

[0065] The test results of the welding performance of the examples and comparative examples are shown in Table 6. The test method of welding performance is as follows: the test bars of the examples and comparative examples are butt welded with the same specification welding wire (specification Ф1.6mm) using the inert gas tungsten inert gas welding method with a line energy of 15kJ / cm. After welding, the samples are ground, polished and etched. The presence of heat-affected zone (HAZ) liquefaction cracks and heat-affected zone stress relaxation cracks is observed in the weld joint. V-notch test samples are taken from the weld joint and subjected to room temperature (20℃) impact test to evaluate the toughness and thus evaluate its welding performance.

[0066] Table 1. Main chemical components (wt%) of the examples and comparative examples

[0067] The impurity content is as follows: Si: ≤0.30%, Co: ≤0.2%, Cu: ≤0.07%, S: ≤0.005%, P: ≤0.008%, O: ≤10ppm, H: ≤1ppm, Pb: ≤0.001%, Sb: ≤0.0025%, Sn: ≤0.0012%, Bi: ≤0.001%, As: ≤0.002%.

[0068] Table 2. Relationship of alloying elements in the examples and comparative examples

[0069] Table 3. Grain size, precipitation, and phase content

[0070] Table 4. Room temperature tensile properties and impact toughness (20°C) of the examples and comparative examples.

[0071] Table 5. High-temperature tensile properties and creep rupture properties (850°C) of the examples and comparative examples.

[0072] Table 6 Welding performance of the examples and comparative examples

[0073] As can be seen from the data in Tables 4-6 above, the nickel-iron-based heat-resistant alloy prepared in the embodiments of the present invention has good room temperature and high temperature mechanical properties, as well as excellent high temperature creep resistance and weldability.

[0074] Figure 5 shows the SEM image of the nickel-iron-based alloy prepared in Example 1. The matrix has good solid solution effect, and the fine Zr(C,N) and a small amount of M6C carbides are distributed relatively evenly and dispersedly with a grain size of 5.0. The above microstructure features together ensure the excellent comprehensive performance of the alloy.

[0075] Comparative Example 1 contained excessively low levels of Cr, while Comparative Example 2 contained excessively low levels of W and Mo. Since Cr, W, and Mo are the main solid solution strengthening elements and carbide M6C forming elements, compared to Examples 1-5, Comparative Examples 1-2, although reducing material costs, showed a significantly reduced degree of solid solution strengthening, insufficient M6C precipitation, larger grain size (all grade 3.0), and significantly reduced tensile strength and yield strength at both room temperature and high temperature. Furthermore, the 850℃ / 50MPa creep rupture life was reduced to below 790 hours.

[0076] Comparative Example 3 contained excessive amounts of W and Mo. Compared to the examples, the W and Mo elements promoted the precipitation of the σ phase, impairing the high-temperature microstructure stability of the alloy and increasing crack susceptibility. This resulted in a reduced creep rupture life, significantly decreased plasticity and toughness, with room temperature elongation dropping to 39% and room temperature reduction of area to 45%. Furthermore, the low diffusion coefficients of W and Mo led to a decline in the weldability of the alloy, with HAZ stress cracking observed and HAZ impact toughness deteriorating during weldability assessment.

[0077] Compared with Example 4, which did not contain Zr, the addition of a suitable amount of Zr can improve the morphology of grain boundary carbides and make them easier to spheroidize, hinder grain boundary migration and refine grains. It can also improve grain boundary strength by increasing the compactness of the grain boundary structure, thereby improving stress concentration and the ductility and toughness of the alloy. The absence of Zr indirectly promotes the precipitation of trace amounts of brittle CrN nitrides at grain boundaries. The creep rupture performance of the alloy under 850℃ / 50MPa conditions decreased to 739h, while the grain size was larger (grain size grade 3.0), and the ductility and toughness at room temperature and high temperature were also reduced.

[0078] Comparative Example 5 had an excessive amount of nitrogen added. The excessive nitrogen promoted the formation of brittle intergranular continuous CrN nitrides in the alloy at 850℃, which significantly reduced the alloy's creep performance and high-temperature strength, and also reduced the HAZ toughness after welding. Figure 6 The SEM image of the nickel-iron-based alloy prepared in Comparative Example 5 shows that, in addition to a small amount of M6C precipitation, CrN nitrides were continuously precipitated along the grain. This continuous precipitation weakened the grain boundary bonding force and significantly reduced the creep performance and high-temperature strength.

[0079] Comparative Examples 6 and 7 were modified with excessively low and excessively high levels of carbon, respectively. The excessively low carbon content in Comparative Example 6 resulted in a low carbide content and a large grain size (grain size grade 2.5), significantly reducing room temperature tensile strength and high-temperature strength, while also causing a certain degree of decrease in creep rupture performance. The excessively high carbon content in Comparative Example 7 resulted in coarse carbide morphology and excessive precipitation. Although fine grains (grain size grade 7.0) were obtained, the solid solution strengthening effect of the alloying elements was weakened, increasing the alloy's susceptibility to HAZ liquefaction cracking and stress relaxation cracking. Furthermore, aggregated M6C carbides appeared in the microstructure of Comparative Example 7 (see...). Figure 7 This is formed during the solidification process of the alloy and could not be dissolved even after solution treatment. Ultimately, the strength of the alloy in Comparative Example 7 was not significantly improved, while its ductility, toughness, and creep performance decreased significantly, and its weldability was also relatively poor.

[0080] Comparative Example 8 reduced the Ni content, significantly decreasing the alloy's solid solution capacity. Excessive carbide precipitation resulted in excessively fine grain size (grain size grade 7.5), leading to a decrease in matrix strength. The finer grains made the grain boundaries significant weak points during creep deformation, resulting in a reduced creep life. Simultaneously, its oxidation resistance and microstructural stability at high temperatures deteriorated, ultimately leading to lower overall mechanical properties compared to Examples 1-5, and the appearance of weld liquefaction cracks.

[0081] The absence of B element in Comparative Example 9 reduced the grain boundary bonding force of the alloy, resulting in a significant decrease in creep life at 850℃ / 50MPa. This will affect the performance stability of the alloy during long-term service at high temperatures, while also slightly reducing the impact toughness.

[0082] The excessive addition of rare earth element Ce in Comparative Example 10 led to a decrease in high-temperature oxidation resistance and a weakening of grain boundary strength. Consequently, the high-temperature strength, plasticity, and creep rupture properties of the alloy in Comparative Example 10 all decreased, the impact toughness after welding also deteriorated, and the hot workability was significantly reduced, while the room temperature strength and plasticity were not significantly affected.

[0083] In Comparative Example 11, no Mn element was added, resulting in a certain degree of reduction in high-temperature strength and creep performance, deterioration in weldability, and the appearance of HAZ liquefaction cracks and poor impact toughness after welding.

[0084] In Comparative Example 12, Zr and N do not conform to the relationship, resulting in the precipitation of Ni5Zr phase, which is detrimental to the weldability of the alloy.

[0085] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A nickel-iron-based heat-resistant alloy, characterized in that, The composition of the nickel-iron-based heat-resistant alloy, by mass percentage, includes: C: 0.025%~0.075%, Ni: 52%~58%, Cr: 17.0%~20.0%, Mo: 7.0%~8.5%, W: 2.0%~3.5%, Mn: 0.4%~1.0%, Al: 0.05%~0.2%, Zr: 0.04%~0.15%, N: 0.005%~0.03%, B: 0.002%~0.006%, Ce: 0.01%~0.03%, with the balance being iron and unavoidable impurity elements.

2. The nickel-iron-based heat-resistant alloy according to claim 1, characterized in that, In the nickel-iron-based heat-resistant alloy, the contents of Cr, Mo and W satisfy the following relationship: 155≤100Cr×(100Mo+50W)≤195, where Cr, Mo and W refer to the mass percentages of the elements Cr, Mo and W in the nickel-iron-based heat-resistant alloy, respectively.

3. The nickel-iron-based heat-resistant alloy according to claim 1, characterized in that, In the nickel-iron-based heat-resistant alloy, the contents of Zr and N satisfy the following relationship: -0.0009+163N<100Zr<-0.00122+720N, where Zr and N refer to the mass percentages of elements Zr and N in the nickel-iron-based heat-resistant alloy, respectively.

4. The nickel-iron-based heat-resistant alloy according to claim 1, characterized in that, The composition of the nickel-iron-based heat-resistant alloy, by mass percentage, includes: C: 0.035%~0.06%, Ni: 54%~57%, Cr: 17.2%~20.0%, Mo: 7.3%~8.5%, W: 2.5%~3.5%, Mn: 0.6%~0.9%, Al: 0.08%~0.15%, Zr: 0.04%~0.15%, N: 0.008%~0.025%, B: 0.003%~0.005%, Ce: 0.011%~0.026%, with the balance being iron and unavoidable impurity elements.

5. The nickel-iron-based heat-resistant alloy according to claim 1, characterized in that, The impurity elements in the nickel-iron-based heat-resistant alloy, by mass percentage, include: Si: ≤0.30%, Co ≤0.2%, Cu: ≤0.07%, S ≤0.005%, P ≤0.008%, O ≤10ppm, H ≤1ppm, Pb ≤0.001%, Sb ≤0.0025%, Sn ≤0.0012%, Bi ≤0.001%, As ≤0.002%.

6. The nickel-iron-based heat-resistant alloy according to any one of claims 1 to 5, characterized in that, The microstructure of the nickel-iron-based heat-resistant alloy mainly consists of austenitic equiaxed grains and dispersed precipitates; the precipitates mainly include M6C and Zr(C,N) carbides.

7. The nickel-iron-based heat-resistant alloy according to claim 6, characterized in that, In the microstructure of the nickel-iron-based heat-resistant alloy, the mass percentage of M6C is 0.20%~0.35%; and the mass percentage of Zr(C,N) is 0.05%~0.15%.

8. A method for preparing a nickel-iron-based heat-resistant alloy according to any one of claims 1 to 7, characterized in that, The preparation method includes the following steps: Step 1: Smelting to obtain ingots; Step 2: Homogenize the ingot; Step 3: Forge the ingot to obtain a forging billet; Step 4: Perform solution treatment on the forging billet to obtain a nickel-iron-based heat-resistant alloy.

9. The preparation method according to claim 8, characterized in that, In step 2, the homogenization process includes: heating the ingot to 1150~1180℃, holding the ingot at that temperature for 24~50h after it is fully heated, and then cooling it to room temperature in the furnace.

10. The preparation method according to claim 8 or 9, characterized in that, In step 3, the initial forging temperature is controlled at 1100~1150℃, and the final forging temperature is controlled at 1000~1010℃.

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