Non-standard-requirement Inconel 718 alloy super-long forged material and manufacturing method thereof

Abstract

The invention provides a non-standard Inconel 718 alloy super-long forged material and a manufacturing method thereof, and the manufacturing method comprises the following steps: preparing an electrode bar through vacuum induction melting, and carrying out electroslag remelting to obtain an electroslag ingot with the diameter of 500 + / -20 mm; carrying out homogenization heat preservation on the electroslag ingot at 1140-1200 DEG C for not less than 120 hours; after being cogged and deformed into square billets, the square billets are forged into round bars with the diameter phi being 180 + / -10 mm and the length being larger than 12000 mm through a high-speed forging machine within the single heating number, and the finish forging temperature is controlled to be larger than or equal to 980 DEG C so as to restrain delta phase precipitation; and then solid solution and aging heat treatment is conducted, the solid solution temperature is 1000 + / -10 DEG C, the aging temperature is 700 + / -10 DEG C, and solid solution for 2-4 hours and aging heat preservation for 6-8 hours are matched to meet the requirement for cross section through burning and structure consistency. The super-long forging material prepared by the method is uniform in chemical component, low in magnification and free of macroscopic defects, carbides in a high-magnification structure are uniformly distributed and free of Laves phases, the room-temperature mechanical property and the intergranular corrosion property can be comprehensively matched, and the method is suitable for manufacturing super-long high-temperature alloy forging materials with non-standard performance requirements.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of alloy manufacturing, in particular to a non-standard Inconel 718 alloy super-long forged material and a manufacturing method thereof. BACKGROUND

[0002] Inconel 718 alloy is a precipitation hardening type of wrought superalloy, which has high tensile strength, creep resistance and good oxidation resistance and corrosion resistance below 650 DEG C, and also has good fatigue strength under cold and hot alternating conditions, so it is widely used in fields such as aero-engine turbine disc, blade, high-temperature fastener, nuclear power equipment, steam turbine and oil field equipment. The alloy can be made into forged material, forged piece, rolled material, plate material, wire material and cold drawn wire material, etc., which has certain processing adaptability; in the prior art, the civil Inconel 718 alloy forged material is usually produced by the process route of vacuum induction smelting combined with electroslag remelting, high-purity electrode rods are obtained by vacuum induction smelting, then electroslag ingots are prepared by electroslag remelting under the condition of protective atmosphere, and then the electroslag ingots are subjected to high-temperature homogenization treatment to promote the diffusion of alloying elements and reduce dendritic segregation. The steel ingot after homogenization treatment is forged into material under certain temperature and holding time conditions, and finally is treated according to the heat treatment system specified in the relevant technical standard and the mechanical properties are detected.

[0003] However, the existing process route is mainly aimed at short-length conventional specification forgings. The process parameters and heat treatment system are usually set based on general technical standards, and the detection items mainly include room temperature tensile properties, room temperature hardness, low temperature impact properties, and high temperature endurance properties, etc. Generally, no explicit requirements are proposed for intergranular corrosion properties. In actual engineering applications, with the diversification and complication of the use environment, some application scenarios propose non-standardized requirements for Inconel 718 alloy forgings, for example, while meeting the requirements of specific size specifications, the forgings are required to achieve comprehensive matching among mechanical properties, grain size, and intergranular corrosion properties. Such requirements often exceed the applicable range of existing general technical standards, especially in the case of significantly increased forging size, the existing mature process is difficult to be directly applied. When it is necessary to prepare ultra-long Inconel 718 alloy forgings with a diameter of more than Φ180 mm and a length of more than 12000 mm, the existing technology faces many technical bottlenecks; the single weight of the finished forgings of this specification is large, and generally an electroslag ingot with a diameter of about Φ500 mm is used as the original blank. However, it is generally believed in the industry that when the diameter of Inconel 718 alloy electroslag ingot exceeds Φ450 mm, the molten pool shape and solidification conditions are difficult to effectively control, and obvious element macrosegregation is easily produced, thereby adversely affecting the metallurgical quality and subsequent processing performance. Secondly, Inconel 718 alloy belongs to age-hardening type high-temperature alloy, and its hot working plasticity rapidly decreases with the decrease of temperature and the increase of precipitated phase. In the process of forging on a quick forging machine, with the extension of forging time and the decrease of blank temperature, the material is prone to produce forging cracks, and the cracks are prone to rapidly expand in the continuous deformation process, resulting in the rejection of the forgings. Therefore, the existing quick forging machine process is generally not suitable for the forming of Inconel 718 alloy forgings with a length of more than 10 m, and the crack control and organization uniformity in the process of ultra-long forging are difficult to guarantee.

[0004] In addition, for Inconel 718 alloy forgings with intergranular corrosion performance requirements, there is a lack of mature experience data for reference in the existing technology; the intergranular corrosion performance is closely related to the alloy chemical composition, the control of precipitated phase in the forging process, and the subsequent heat treatment system. The existing conventional heat treatment process mainly focuses on the acquisition of strength and plasticity, and lacks targeted design for the balance between grain size and intergranular corrosion performance, and it is difficult to meet the intergranular corrosion performance requirements while ensuring the mechanical properties.

[0005] Therefore, in the aspects of ultra-large specification electroslag ingot metallurgical quality control, ultra-long Inconel 718 alloy forging feasibility, and non-standard mechanical property and intergranular corrosion performance matching, the existing technology still has certain limitations and needs to be further improved and perfected. SUMMARY

[0006] One of the technical problems to be solved by the present application is to provide a manufacturing method of non-standard Inconel 718 alloy super-long forged material, so as to solve the problem that it is difficult to match the metallurgical quality control of large-diameter electroslag ingot, the forming stability of super-long forged material, and the mechanical properties and intergranular corrosion performance in the prior art.

[0007] To overcome the defects of the prior art, the present application provides a manufacturing method of non-standard Inconel 718 alloy super-long forged material, comprising the following steps: S1: preparing a large-diameter electroslag ingot: preparing an electrode rod by vacuum induction melting, and then performing electroslag remelting, wherein the melting speed of the electroslag remelting is controlled to be 220-300 kg / h, so as to obtain an Inconel 718 alloy electroslag ingot with a diameter of 500±20 mm; S2: high-temperature homogenization: homogenizing the electroslag ingot at 1140-1200℃ for not less than 120 hours; S3: super-long forging breakdown and forming: forging the homogenized electroslag ingot, first deforming the electroslag ingot into a square billet by breakdown heating, and then forging the square billet into a round bar with a diameter of Φ180±10 mm and a length of more than 12000 mm in a single heating, wherein the finish forging temperature of the final forming heating is controlled to be not less than 980℃; S4: performance matching heat treatment: sequentially performing solid solution treatment and aging treatment on the round bar, wherein the temperature of the solid solution treatment is 1000±10℃, and the temperature of the aging treatment is 700±10℃.

[0008] Compared with the prior art, the manufacturing method of the non-standard Inconel 718 alloy super-long forged material has the following advantages: the manufacturing method of the non-standard Inconel 718 alloy super-long forged material realizes the comprehensive balance between the microstructure control, mechanical properties and intergranular corrosion performance under the condition of super-long size through the systematic collaborative design of the smelting, forging and heat treatment process parameters, in step S1, by limiting the melting speed of electroslag remelting to 220-300 kg / h and combining with the subsequent long-time high-temperature homogenization treatment, the electroslag ingot with a diameter of 500±20 mm can still obtain a relatively stable molten pool shape and uniform chemical composition distribution, thereby reducing the risk of element macrosegregation in the large-diameter electroslag ingot from the source, and providing a metallurgical quality basis for subsequent super-long forging forming; in the forging stage of step S3, the forming of the super-long round bar is completed in a single material forming fire, and the finish forging temperature is controlled to be not lower than 980 DEG C, thereby effectively inhibiting the precipitation of the delta phase of the Inconel 718 alloy in the forging process, ensuring the hot working plasticity of the material under the condition of large deformation, and avoiding the problems of cracks and uneven microstructure commonly existing in the super-long forged material; and in the heat treatment stage of step S4, the process combination of 1000±10 DEG C solid solution treatment and 700±10 DEG C aging treatment is adopted, the second phase precipitated in the forging process is fully dissolved through the solid solution treatment, the grain size is not obviously coarsened, and the grain boundary state is improved; and then the uniform precipitation of the strengthening phase is promoted through the moderate aging treatment, so that the synergistic effect between the strength, plasticity and corrosion resistance is formed, compared with the performance not comprehensive problem caused by the separate adjustment of the conventional high-temperature solid solution or high-temperature aging process, the solid solution and aging treatment adopted by the present application can significantly reduce the intergranular corrosion rate while keeping the grain size meeting the requirements, and simultaneously improve the tensile strength and elongation after fracture; the manufacturing method of the present application effectively solves the problems of difficult control of the metallurgical quality of the large-size electroslag ingot, insufficient stability of the super-long forging forming, and difficult consideration of the mechanical properties and intergranular corrosion performance in the prior art, and is suitable for the manufacturing of high-temperature alloy forged material with non-standard performance requirements.

[0009] In a possible implementation, in the step S1, the vacuum induction melting includes: S11: melting period: under the condition of a vacuum degree ≤1 Pa, melting the carbon-containing raw material and nickel, chromium, molybdenum, niobium and iron raw materials; S12: refining period: refining the molten steel under the condition of a vacuum degree ≤1 Pa; S13: alloying and pouring: adding aluminum and titanium for alloying, and then pouring to obtain an electrode rod under the condition of a vacuum degree ≤1.0 Pa.

[0010] Compared with the prior art, the above technical scheme has the advantages that, in the step S1, the vacuum degree is controlled to be not more than 1 Pa during the melting period and the refining period, and the vacuum degree of the pouring environment is controlled to be not more than 1.0 Pa during the alloying and pouring stages, which is beneficial to reduce the gas content in the molten steel and reduce the formation of non-metallic inclusions, the appropriate amount of carbon is added during the melting period, CO is generated by the carbon-oxygen reaction under vacuum, the solubility of CO in the molten steel is small, and CO is easily removed, thereby playing a good deoxidizing effect, and since the molten steel is smelted and poured under high vacuum conditions, secondary pollution can be effectively inhibited, the obtained electrode rod has high purity and stable chemical composition, and a good metallurgical quality basis is provided for subsequent electroslag remelting to prepare a large-diameter electroslag ingot.

[0011] In a possible implementation, in the step S11, the melting temperature is 1520±10℃; and in the step S12, the refining temperature is 1480±10℃, and the refining time is greater than or equal to 1 hour.

[0012] Compared with the prior art, the above implementation has the advantages that, by controlling the melting temperature in the step S11 to be 1520±10℃ and controlling the refining temperature in the step S12 to be 1480±10℃ and the refining time to be not less than 1 hour, the molten steel can be fully melted and refined, and at the same time, excessive loss of alloying elements can be avoided, and under the above temperature and time conditions, the gas and inclusion contents in the molten steel can be reduced, and the composition distribution of the electrode rod is more uniform, thereby being beneficial to improve the molten pool stability in the subsequent electroslag remelting process.

[0013] In a possible implementation, in the step S13, the operation of detecting the composition of the molten steel is further included between the addition of aluminum and titanium for alloying and the pouring, and the pouring is performed at a temperature of 1500±10℃.

[0014] Compared with the prior art, in the step S13, the composition of the molten steel is detected after the addition of aluminum and titanium for alloying and before pouring, and the pouring temperature is controlled to be 1500±10℃, which can ensure that the chemical composition of the electrode rod meets the design requirements, and the pouring temperature of 1500±10℃ can ensure that the molten steel has good fluidity and stable solidification, thereby reducing the generation of defects such as shrinkage holes and porosity, and improving the internal quality stability of the electrode rod.

[0015] In a possible implementation, in the step S2, the homogenization treatment includes: charging at a temperature not higher than 600℃, holding at a temperature of 1140-1200℃ for not less than 120 hours after the temperature is raised, then lowering the temperature to 1100±10℃ and holding, and then discharging.

[0016] Compared with the prior art, in the embodiment, the electric ingot is heated more uniformly in the temperature rising and high temperature stages by charging at a temperature not higher than 600 DEG C and long time holding at 1140-1200 DEG C for not less than 120 hours in the step S2, which is beneficial to the full diffusion of alloy elements and the elimination of dendritic segregation; then the electric ingot is discharged after holding at 1100±10 DEG C, which avoids the uneven structure caused by temperature mutation; the embodiment is beneficial to obtaining large-diameter electric ingot with uniform composition and structure, and provides a stable structure basis for subsequent ultra-long forging forming, and reduces the adverse effect of macrosegregation on the performance of finished product.

[0017] In a possible embodiment, in the step S3, the blooming pass includes upsetting and elongating deformation of the electric ingot to obtain a square billet with a cross section of (280-320) mm x (280-320) mm; the finish pass is completed by a fast forging machine, and the two ends of the square billet are held by two operating cars of the fast forging machine for forging. The forging deformation process of the finish pass includes first forging the square billet into an intermediate billet with a cross section of 170-174 mm, then chamfering into a polygon, and finally rounding into the round bar.

[0018] Compared with the prior art, by adopting the above technical scheme, the ultra-long billet is subjected to more uniform stress in the forging process by first performing upsetting and elongating deformation of the electric ingot to obtain a square billet with a cross section in the above range, and holding the two ends of the square billet by two operating cars of a fast forging machine in the finish pass; and by further sequentially completing square billet deformation, chamfering and rounding in a single finish pass, the forging time is shortened while ensuring a large deformation, which is beneficial to promoting full recrystallization and refining grains, thereby improving the consistency and forming stability of the overall structure of the ultra-long forged material.

[0019] In a possible embodiment, the step S3 further includes: after forging into a round bar, cutting off a part with a length not less than 200 mm of the two ends of the round bar in contact with the tongs to remove the heat affected zone with a final forging temperature lower than 980 DEG C.

[0020] Compared with the prior art, by adopting the above technical scheme, the area of the part with a length not less than 200 mm of the two ends of the round bar in contact with the tongs is subjected to rapid temperature drop in the forging process, which is prone to produce abnormal structure or precipitate harmful phases, and the embodiment avoids the influence of the low final forging temperature area on the structure and performance of the finished product, thereby ensuring the stability of the structure and performance of the ultra-long forged material in the length direction.

[0021] In a possible implementation, in the step S4, the holding time of the solid solution treatment is 2-4 hours, followed by air cooling; the holding time of the aging treatment is 6-8 hours, followed by air cooling.

[0022] Compared with the prior art, the above technical solution controls the holding time of the solid solution treatment to be 2-4 hours and air cools, which can make the second phase precipitated in the forging process sufficiently dissolve, while avoiding the grains to grow significantly due to the too long holding time; then the holding time of the aging treatment is controlled to be 6-8 hours and air cools, which is beneficial to the uniform precipitation of the strengthening phase in the matrix, so as to obtain a stable strengthening effect; the present application also realizes the comprehensive balance between the mechanical properties and the intergranular corrosion performance by the cooperation control of the solid solution treatment time and the aging treatment time, while ensuring the grain size to be in a reasonable range, the strength and plasticity are taken into account, and the influence of the unfavorable phase at the grain boundary on the intergranular corrosion performance is reduced, without separately pursuing the strength or corrosion resistance, the comprehensive balance between the mechanical properties and the intergranular corrosion performance is realized.

[0023] Another technical problem to be solved by the present application is to provide an Inconel 718 alloy super-long forged material with non-standard requirements, so as to solve the problem that the super-long specification forged material is difficult to meet the size requirements while taking into account the mechanical properties and intergranular corrosion performance in the prior art.

[0024] In order to overcome the defects of the above prior art, the present application provides an Inconel 718 alloy super-long forged material, which is prepared by the above manufacturing method, and has a diameter of Φ180±10 mm and a length of not less than 12000 mm, and meets the following performance indexes: yield strength σ0.2 is 750-850 MPa, tensile strength σb is not less than 1100 MPa, elongation after fracture δ is not less than 30%, and the intergranular corrosion rate tested according to ASTM G28 A method is not higher than 0.06 mm / month.

[0025] In a possible implementation, the chemical composition meets the following requirements in terms of mass percentage: C≤0.05%, Cr: 17.00-21.00%, Mo: 2.80-3.30%, Nb: 4.75-5.50%, Ti: 0.75-1.15%, Al: 0.30-0.70%, Ni: 50.00-55.00%, Mn≤0.35%, Si≤0.35%, S≤0.015%, P≤0.015%, B≤0.006%, Cu≤0.30%, and the balance is Fe and inevitable impurities.

[0026] Compared with existing technologies, the Inconel 718 alloy ultra-long forging of this invention has the following advantages: By establishing a coordinated relationship between the metallurgical quality control of large-diameter electroslag ingots, ultra-long forging, and heat treatment processes, the forging can still achieve stable and uniform microstructure and properties even with a length of not less than 12000 mm. Through the overall design and compositional control of the forging size, chemical composition, and heat treatment regime, the forging maintains high tensile strength while also possessing good plasticity and meeting intergranular corrosion resistance requirements, thus avoiding the common problems of insufficient strength and plasticity or inadequate corrosion resistance in existing ultra-long Inconel 718 alloy forgings. This ultra-long forging can meet the non-standard performance requirements of users for specific application environments, expanding the application range of Inconel 718 alloy forgings in the ultra-long dimension field. Detailed Implementation

[0027] First, those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention. Those skilled in the art can make adjustments as needed to adapt to specific application scenarios.

[0028] This invention provides a method for manufacturing non-standard Inconel 718 alloy ultra-long forgings, comprising the following steps: S1: Preparation of large-diameter electroslag ingots: Electrode rods are prepared by vacuum induction melting, followed by electroslag remelting, wherein the melting rate of electroslag remelting is controlled at 220-300 kg / h, to obtain Inconel 718 alloy electroslag ingots with a diameter of 500±20 mm. S2: High-temperature homogenization: The electroslag ingot is homogenized by holding it at 1140-1200℃ for no less than 120 hours. S3: Ultra-long forging billet opening and forming: The electroslag ingot after homogenization treatment is forged. First, it is deformed into a square billet through a billet opening fire. Then, the square billet is forged into a round bar with a diameter of Φ180±10 mm and a length greater than 12000 mm in a single fire. The final forging temperature of this finished product fire is controlled to be no less than 980℃. S4: Performance matching heat treatment: The round bar is subjected to solution treatment and aging treatment in sequence, wherein the solution treatment temperature is 1000±10℃ and the aging treatment temperature is 700±10℃.

[0029] As a preferred embodiment, step S1, the vacuum induction melting includes: S11: Melting period: Under vacuum conditions ≤1 Pa, carbon-containing raw materials are melted with nickel, chromium, molybdenum, niobium and iron raw materials; S12: Refining period: Refining molten steel under vacuum conditions ≤1 Pa; S13: Alloying and casting: Aluminum and titanium are added for alloying, and then the electrode rod is cast under vacuum conditions ≤1.0 Pa to obtain the electrode rod.

[0030] As a preferred embodiment, in step S11, the melting temperature is 1520±10℃; in step S12, the refining temperature is 1480±10℃, and the time is ≥1 hour.

[0031] As a preferred embodiment, in step S13, the process of adding aluminum and titanium for alloying and casting includes an operation of detecting the composition of the molten steel, and the casting condition is to cast at a temperature of 1500±10℃.

[0032] As a preferred embodiment, in step S2, the homogenization process includes: loading the furnace at a temperature not exceeding 600°C, heating it to 1140-1200°C and holding it at that temperature for at least 120 hours, then cooling it to 1100±10°C and holding it at that temperature, and then unloading it from the furnace.

[0033] As a preferred embodiment, in step S3, the billet preparation process includes upsetting and drawing deformation of the electroslag ingot to obtain a square billet with a cross-section of (280-320) mm × (280-320) mm; the finished product preparation process is completed using a high-speed forging machine, and the two ends of the square billet are forged by linking two operating carriages of the high-speed forging machine. The forging and deformation process of the finished product includes: first forging the square billet into an intermediate billet with a cross-section of 170-174 mm square, then chamfering it into a polygonal shape, and finally rolling it into a round bar.

[0034] As a preferred embodiment, step S3 further includes: after forging into a round bar, cutting off the portion of the round bar that is in contact with the clamp at both ends with a length of not less than 200 mm, in order to remove the heat-affected zone where the final forging temperature is below 980°C.

[0035] As a preferred embodiment, in step S4, the heat treatment time for the solution treatment is 2-4 hours, followed by air cooling; the heat treatment time for the aging treatment is 6-8 hours, followed by air cooling.

[0036] This invention provides an Inconel 718 alloy ultra-long forging, which is manufactured by the above-mentioned method, and has a diameter of Φ180±10 mm and a length of not less than 12000 mm, and meets the following performance indicators: yield strength σ0.2 of 750-850 MPa, tensile strength σb of not less than 1100 MPa, elongation after fracture δ of not less than 30%, and intergranular corrosion rate tested according to ASTM G28 A method of not more than 0.06 mm / month.

[0037] As a preferred embodiment, its chemical composition, by mass percentage, satisfies the following: C ≤ 0.05%, Cr: 17.00-21.00%, Mo: 2.80-3.30%, Nb: 4.75-5.50%, Ti: 0.75-1.15%, Al: 0.30-0.70%, Ni: 50.00-55.00%, Mn ≤ 0.35%, Si ≤ 0.35%, S ≤ 0.015%, P ≤ 0.015%, B ≤ 0.006%, Cu ≤ 0.30%, with the balance being Fe and unavoidable impurities.

[0038] In the research process of this invention, ProCAST software was first used to simulate and analyze the solidification process of Inconel 718 alloy electroslag ingots with a diameter of Φ500 mm under different process parameters to assess the potential metallurgical quality risks during electroslag remelting. Subsequently, DEFORM software was used to simulate and analyze the temperature field and stress-strain distribution of the billet during forging, focusing on the process feasibility of forging ultra-long forgings with a diameter of Φ180 mm and a length greater than 12000 mm. Through the above simulation analysis, the metallurgical quality control of large-diameter electroslag ingots and the forging process of ultra-long forgings were predicted, thus basically determining the feasibility of using Φ500 mm electroslag ingots to produce Inconel 718 alloy forgings with a diameter of Φ180 × >12000 mm. Based on this, actual production was carried out according to the determined process route, and ultra-long forging products were successfully prepared. Subsequently, samples were taken from the forgings and different heat treatment processes were explored under laboratory conditions. By testing mechanical properties and intergranular corrosion resistance, the final heat treatment method suitable for this specification of forging was determined. The Inconel 718 alloy Φ180×>12000 mm forgings produced using the manufacturing method of this invention meet the technical standard requirements in terms of physical and chemical properties, and can be successfully delivered.

[0039] To further illustrate the present invention, the following specific embodiments, incorporating concrete data and experimental methods, are provided: Example 1 This embodiment provides a method for manufacturing non-standard Inconel 718 alloy ultra-long forgings. The process includes: selecting raw materials, vacuum induction, electroslag remelting, ingot homogenization, forging into finished products, sampling, physical and chemical property testing, finished product heat treatment, sampling, physical and chemical property testing, finishing, and length setting.

[0040] S1: Preparation of large-diameter electroslag ingots S11: Melting period Raw materials such as carbon, nickel, chromium, molybdenum, niobium, and iron are loaded into a vacuum induction furnace. The furnace is energized and evacuated. When the vacuum level inside the furnace is less than 1 Pa, the furnace is energized to melt the materials. The temperature T1 is measured to be 1520±10 ℃, which is considered as the complete melting of the raw materials.

[0041] S12: Refining Period The temperature of the molten steel is reduced to 1480±10 ℃, and the refining period begins. Refining is carried out under a vacuum of ≤1 Pa, and electromagnetic stirring is performed appropriately to cause the gas and non-metallic inclusions in the molten steel to float to the surface. The gas is removed and the non-metallic inclusions float to the surface of the molten steel, thereby reducing the gas and inclusion content in the molten steel. High vacuum smelting is maintained throughout the refining period, and the refining time is not less than 1 hour.

[0042] S13: Alloying and Casting After refining, the power was increased and aluminum and titanium were added for alloying. After alloying for 40 minutes, samples were taken to analyze the chemical composition of the molten steel. If the chemical composition met the requirements, the temperature was measured at T3 = 1500 ± 10 ℃ to prepare for tapping. The runner chamber and ingot mold chamber were evacuated to a high vacuum state (≤1.0 Pa), and casting was carried out under vacuum conditions to obtain electrode rods with a diameter of Φ420 mm. After casting, the electrode rods were cooled under vacuum conditions for no less than 2 hours before demolding and air-cooled to room temperature for later use. The measured chemical composition (wt%) is shown in Table 1. Table 1 Chemical composition (wt%) of Inconel 718 alloy electrode rod in Example 1 The aforementioned S2 electrode rod was subjected to electroslag remelting at a melting rate of 220 kg / h-300 kg / h to obtain an electroslag ingot with a diameter of Φ500 mm.

[0043] In this invention, prior to actual manufacturing, computer software was used to simulate and analyze the electroslag remelting process. The results showed that, under reasonable control of the remelting rate, a stable and relatively flat molten pool could be formed. The width of its two-phase region was similar to that of a conventional electroslag ingot with a diameter of Φ360 mm. Theoretically, this condition suggests that significant macroscopic elemental segregation is unlikely to occur. Based on these simulation results, an electroslag ingot with a diameter of Φ500±20 mm was selected as the initial billet for the production of this specification of ultra-long forging. Actual production and testing results showed that no macroscopic segregation was observed in the low-magnification microstructure of the forging at both the head and tail ends, verifying the rationality of the selected process parameters. Thus, the stable preparation of an Inconel 718 alloy electroslag ingot with a diameter of Φ500±20 mm was achieved, providing reliable metallurgical quality assurance for the subsequent successful production of ultra-long forgings.

[0044] S2: High-temperature homogenization After cooling, the electroslag ingot is reheated for high-temperature homogenization treatment: it is loaded into the furnace at a temperature not exceeding 600℃, heated to 1140–1200℃ after 6 hours and held for 120 hours, then cooled to 1100±10℃ and held for an appropriate time before being taken out of the furnace for forging.

[0045] S3: Extra-long forging blanking and forming Billet preparation: The homogenized electroslag ingot is taken out of the furnace and forged: the first upsetting is to 1 / 2 of the original steel ingot height and drawn to a 500×500 mm square billet, and then put back into the furnace for heat preservation for 1 hour; the second upsetting is to draw to a 300×300×4050 mm square billet, and Φ200×500 mm long clamps are made at both ends before putting it back into the furnace for a further heating time of 1.5 hours.

[0046] Forming: Set the parameters of the high-speed forging machine operating carriage to keep the two operating carriages linked and at the same frequency; after the billet is taken out of the furnace, the two operating carriages hold the clamps at both ends for forging. First, continue forging to a 172×172 mm square billet, then bevele it into a sixteen-cornered shape, and finally roll it into an ultra-long forging material of Φ180×12800 mm. The final forging temperature is ≥980 ℃; after forging, air cool to room temperature.

[0047] Sampling (for finished product inspection): After cutting off the handle, cut a 200 mm long sample from one end, leaving a 12600 mm long remaining sample.

[0048] In this invention, the core of the forging process using a high-speed forging mill lies in the coordinated control of the billet opening method, deformation amount, and final forging temperature. During the preparation process, the billet opening stage of the forging adopts a one-time upsetting and drawing process to open the steel ingot to a square billet of (280-320) mm × (280-320) mm, with a forging ratio of 6.23, which can fully break and eliminate the as-cast structure, laying the foundation for subsequent microstructure refinement. In the forging and forming stage, a one-fire forming process is adopted, with a single-fire deformation amount of more than 70%, which is conducive to sufficient recrystallization during the forging process, thereby obtaining a fine and uniform grain structure. Meanwhile, controlling the final forging temperature above 980 °C is crucial to prevent the precipitation of the δ phase in Inconel 718 alloy during forging, as the precipitation of the δ phase adversely affects the material's intergranular corrosion resistance. This invention employs thermal simulation analysis to study the temperature field changes during the deformation process in the forging process design stage. Simulation results show that when the finished billet is tapped at approximately 1100 °C and forged on a 5000-ton high-speed forging mill, under smooth operation, the final forging temperature at the center of the forging can be controlled at around 1020 °C, and the surface final forging temperature is approximately 980 °C. However, in the areas near the clamping jaws, due to faster heat dissipation, the final forging temperature may drop to 960–970 °C. Based on the above analysis results, an end allowance is reserved in the design of the forging length. After completing two inspection samplings, the part longer than the diameter of the forging is cut off at the other end to remove the heat-affected zone that may affect the stability of the structure due to the low final forging temperature. This ensures that the effective part of the final forging temperature of the delivered forging is not lower than 980 ℃, avoiding the precipitation of δ phase. Through the above forging process design, this invention achieves stable forming of ultra-long Inconel 718 alloy forgings without the need for traditional ultra-long forging insulation measures such as soft sleeves. This not only simplifies the operation process and reduces the impact on the environment and personnel health, but also successfully produces ultra-long forgings by linking two operating carriages of a 5000-ton high-speed forging machine. It has good process practicality and innovation.

[0049] S4: Performance Matching Heat Treatment Taking into account the heat treatment process and the heat treatment time of Φ180 mm bars, this embodiment adopts the following finished product heat treatment regime for the remaining bars: solution treatment at 1000 ℃ for 3 hours followed by air cooling; aging at 700 ℃ for 7 hours followed by air cooling.

[0050] Post-processing: The remaining Φ180×12400 mm forging material is machined to Φ170×12400 mm; then a 400 mm length is cut from one end of the bar to obtain the finished bar Φ170×12000 mm.

[0051] To further illustrate the synergistic effect of solution treatment, aging temperature, and time in this invention, comparative examples are also provided. Specifically, after forging the material in step S3 of Example 1, samples were taken and subjected to different heat treatments to compare and analyze the effects of different heat treatment process parameters on the microstructure and properties of Inconel 718 alloy forgings. Samples were cut from the forgings, and without changing the smelting and forging processes, only the heat treatment process parameters were adjusted. Multiple sets of comparative test conditions were set: by changing the solution treatment temperature while keeping the solution treatment holding time constant, the grain size and intergranular corrosion performance of the samples were compared and tested, and the test results are shown in Table 2. Simultaneously, under the condition of maintaining a consistent solution treatment process, by changing the aging treatment temperature, the room temperature tensile properties and grain size of the samples were compared and tested, and the test results are shown in Table 3.

[0052] The specific operations corresponding to Table 2 are as follows: Take samples from the samples obtained in step S3 above by wire cutting, and take 5 room temperature tensile test samples (Φ20mm), 5 intergranular corrosion test samples (4*30*40mm), and 5 grain size test samples (Φ20mm). Set up 1 grain size test sample and 1 intergranular corrosion test sample as a group. After treating them according to the heat treatment regime in Table 2 below, perform the corresponding tests. Among them, FeCl3 etchant is used for grain size test, and the intergranular corrosion test is performed according to ASTM G28 A method, with boiling ferric sulfate and 50% sulfuric acid as etchants, and the corrosion time is 24 hours.

[0053] Table 2. Heat treatment process test data (grain size, intergranular corrosion) The room temperature tensile specimens from step S3 were heat-treated according to the process in Table 3, and the mechanical properties were tested according to ASTM E8. The results are shown in Table 3 below: Table 3. Test data of heat treatment process (mechanical properties) Based on the test results in Tables 2 and 3, and considering that this product is an ultra-long forging with a diameter of approximately Φ180±10 mm, the heat treatment of the finished product must meet the requirements of temperature uniformity and microstructure consistency in the cross-sectional direction. Therefore, a comprehensive analysis and determination of the heat treatment process parameters were conducted. According to the influence of different solution treatment processes on grain size and intergranular corrosion resistance shown in Table 2, it can be seen that when the solution temperature is 960 ℃ and 980 ℃, the grain size of the samples is grade 8, but the intergranular corrosion rates are 0.077 mm / month and 0.065 mm / month, respectively, indicating relatively poor intergranular corrosion resistance. When the solution temperature is increased to 1000 ℃, the grain size of the samples is grade 7, and the intergranular corrosion rate decreases to 0.055 mm / month. While maintaining a qualified grain size, the intergranular corrosion resistance is significantly improved. Further increasing the solution temperature to 1020 ℃ and 1040 ℃, although the intergranular corrosion rate further decreases, the grain size decreases to grades 5.5 and 4, respectively. At this stage, a significant grain coarsening trend emerges, which is detrimental to obtaining a more balanced overall mechanical property. Therefore, in selecting the solution treatment process, priority should be given to choosing a solution temperature range that can simultaneously meet the requirements for grain size and intergranular corrosion resistance, with the optimal solution temperature being 1000 ℃.

[0054] Based on the influence of different aging processes on room temperature tensile properties shown in Table 3, under the condition of solution treatment at 1000 ℃ × 1 hour with air cooling, as the aging temperature gradually increased from 650 ℃ to 740 ℃, the yield strength and tensile strength of the samples generally showed an upward trend, while the elongation after fracture gradually decreased from 52% to 31%, indicating that as the aging temperature increased, the material strength increased while the plasticity gradually decreased. Considering the matching relationship between strength and plasticity, at an aging temperature of 700 ℃, the sample obtained σ 0.2 With a performance combination of 791 MPa, σb of 1159 MPa, and elongation after fracture of 34.5%, it can maintain good plasticity at a high strength level. Therefore, the optimal aging temperature is determined to be 700 ℃.

[0055] The experimental results shown in Tables 2 and 3 above are based on the sample-level heat treatment conditions, corresponding to small sample sizes. Solution treatment and holding for 1 hour followed by aging for 5 hours are sufficient to achieve cross-sectional heat treatment. However, the finished product is an ultra-long bar with a diameter of approximately Φ180 mm and a larger cross-sectional size. If the sample-level holding time is used, it is easy to cause uneven temperature distribution in the cross-section, which in turn causes differences in the degree of resolution and precipitation along the cross-sectional direction, affecting the stability of grain size, mechanical properties, and intergranular corrosion resistance. Based on the above considerations, the solution treatment holding time in the heat treatment of the finished product is extended from 1 hour for the sample level to 3 ± 1 hours to ensure that the entire cross-section fully reaches the solution temperature and that the second phase precipitated during forging is fully resolution. At the same time, the aging holding time is extended from 5 hours for the sample level to 7 ± 1 hours to ensure that the strengthening phase precipitates more uniformly in the cross-section, thereby obtaining a stable strength-plasticity matching relationship and taking into account intergranular corrosion resistance.

[0056] Therefore, considering the selection criteria for solution treatment temperature, aging temperature, and the process requirements for through-firing of Φ180 mm finished bar sections, the heat treatment process for the finished product in this invention is as follows: solution treatment at 1000±10 ℃ for 2-4 hours; aging at 700±10 ℃ for 6-8 hours followed by air cooling.

[0057] After S4 heat treatment, a 200 mm long sample was cut from the head of the forging for physical and chemical property testing: one room temperature tensile test specimen (Φ20 mm), one intergranular corrosion test specimen (4×30×40 mm), and one chemical composition test specimen (30×30×30 mm). The remaining length was 12400 mm. The chemical composition (wt%) of the S4 sample was tested, and the results of the physical and chemical property testing of the finished product are shown in Tables 4 and 5 below. The microstructure results at low and high magnification are shown in Table 6.

[0058] Table 4 Chemical composition of finished product Table 5 Mechanical properties and intergranular corrosion Table 6. Results of low-magnification and high-magnification tissue analysis As shown in Tables 4 to 6, the Inconel 718 alloy ultra-long forgings produced using the manufacturing method described in this embodiment of the invention have all elements of their chemical composition stably controlled within the technical requirements range. This indicates that, through the combination of vacuum induction melting, electroslag remelting, and high-temperature homogenization processes, the manufacturing method of this invention can achieve uniformity and stability of alloy composition under large-diameter electroslag ingot conditions. Furthermore, after solution treatment at 1000℃ for 3 hours and aging at 700℃ for 7 hours in the manufacturing method, the forgings simultaneously achieved high yield strength and tensile strength at room temperature while maintaining good plasticity. The grain size reached grade 7, and the intergranular corrosion rate was less than 0.06 mm / month, indicating that the forgings under this process achieved a good balance between strength, plasticity, and resistance to intergranular corrosion. Simultaneously, no defects such as segregation, porosity, inclusions, or cracks were observed in the low-magnification microstructure, while the carbides were uniformly distributed and no Laves phase was observed in the high-magnification microstructure, further verifying the effectiveness of this embodiment in microstructure control. As can be seen from the aforementioned comparative data, while simply increasing the solution temperature helps reduce the intergranular corrosion rate, it leads to significant grain coarsening. Conversely, simply increasing the aging temperature, while improving strength, comes at the cost of sacrificing plasticity. In contrast, this invention, by controlling the solution temperature at 1000℃ and combining it with a 700℃ aging process, avoids excessive grain growth while ensuring the full dissolution of the second phase precipitated during forging and achieving uniform precipitation of the strengthening phase during the aging stage. This achieves a comprehensive balance between grain size, strength, plasticity, and intergranular corrosion resistance. Therefore, the technical principle of this invention lies in solving the technical problem in existing technologies where ultra-long Inconel 718 alloy forgings struggle to balance mechanical properties and intergranular corrosion resistance through the coordinated control of smelting purity, final forging temperature, forging conditions, and solution and aging process parameters. It offers significant advantages such as stable microstructure, good performance matching, and suitability for manufacturing ultra-long forgings with non-standard requirements.

[0059] In the description of this invention, the references to "one embodiment," "some embodiments," "in this embodiment," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0060] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for manufacturing non-standard Inconel 718 alloy ultra-long forgings, characterized in that, Includes the following steps: S1: Preparation of large-diameter electroslag ingots: Electrode rods are prepared by vacuum induction melting, followed by electroslag remelting, wherein the melting rate of electroslag remelting is controlled at 220-300 kg / h, to obtain Inconel 718 alloy electroslag ingots with a diameter of 500±20 mm. S2: High-temperature homogenization: The electroslag ingot is homogenized by holding it at 1140-1200℃ for no less than 120 hours; S3: Ultra-long forging billet opening and forming: The electroslag ingot after homogenization treatment is forged. First, it is deformed into a square billet through a billet opening fire. Then, the square billet is forged into a round bar with a diameter of Φ180±10 mm and a length greater than 12000 mm in a single fire. The final forging temperature of this finished product fire is controlled to be no less than 980℃. S4: Performance matching heat treatment: The round bar is subjected to solution treatment and aging treatment in sequence, wherein the solution treatment temperature is 1000±10℃ and the aging treatment temperature is 700±10℃.

2. The manufacturing method according to claim 1, characterized in that, In step S1, the vacuum induction melting includes: S11: Melting period: Under vacuum conditions ≤1 Pa, carbon-containing raw materials are melted with nickel, chromium, molybdenum, niobium and iron raw materials; S12: Refining period: Refining molten steel under vacuum conditions ≤1 Pa; S13: Alloying and casting: Aluminum and titanium are added for alloying, and then the electrode rod is cast under vacuum conditions ≤1.0 Pa to obtain the electrode rod.

3. The manufacturing method according to claim 2, characterized in that, In step S11, the melting temperature is 1520±10℃; in step S12, the refining temperature is 1480±10℃ and the time is ≥1 hour.

4. The manufacturing method according to claim 2, characterized in that, In step S13, the process between alloying with aluminum and titanium and casting includes detecting the composition of the molten steel, and the casting is carried out at a temperature of 1500±10℃.

5. The manufacturing method according to claim 1, characterized in that, In step S2, the homogenization process includes: loading the furnace at a temperature not exceeding 600°C, heating it to 1140-1200°C and holding it at that temperature for at least 120 hours, then cooling it to 1100±10°C and holding it at that temperature, and then unloading it from the furnace.

6. The manufacturing method according to claim 1, characterized in that, In step S3, the billet preparation process includes upsetting and drawing deformation of the electroslag ingot to obtain a square billet with a cross-section of (280-320) mm × (280-320) mm; the finished product preparation process is completed by a high-speed forging machine, and the two ends of the square billet are forged by linking the two operating carriages of the high-speed forging machine. The forging and deformation process of the finished product includes: first forging the square billet into an intermediate billet with a cross-section of 170-174 mm square, then chamfering it into a polygonal shape, and finally rolling it into a round bar.

7. The manufacturing method according to claim 1, characterized in that, Step S3 further includes: after forging into a round bar, cutting off the portion of the round bar that is in contact with the clamp at both ends with a length of not less than 200 mm, in order to remove the heat-affected zone where the final forging temperature is below 980°C.

8. The manufacturing method according to claim 1, characterized in that, In step S4, the heat treatment time for the solution treatment is 2-4 hours, followed by air cooling; the heat treatment time for the aging treatment is 6-8 hours, followed by air cooling.

9. An ultra-long forged Inconel 718 alloy, characterized in that, It is manufactured by any one of the manufacturing methods described in claims 1 to 8, and has a diameter of Φ180±10 mm, a length of not less than 12000 mm, and meets the following performance indicators: yield strength σ0.2 of 750-850 MPa, tensile strength σb of not less than 1100 MPa, elongation after fracture δ of not less than 30%, and intergranular corrosion rate tested according to ASTM G28A method of not more than 0.06 mm / month.

10. The Inconel 718 alloy ultra-long forging according to claim 9, characterized in that, Its chemical composition, by mass percentage, meets the following requirements: C≤0.05%, Cr: 17.00-21.00%, Mo: 2.80-3.30%, Nb: 4.75-5.50%, Ti: 0.75-1.15%, Al: 0.30-0.70%, Ni: 50.00-55.00%, Mn≤0.35%, Si≤0.35%, S≤0.015%, P≤0.015%, B≤0.006%, Cu≤0.30%, with the balance being Fe and unavoidable impurities.

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