A stress relief annealing method for large-size nickel-iron-based high-temperature alloy induction electrodes

Abstract

This invention discloses a stress-relief annealing method for large-size induction electrodes made of nickel-iron-based superalloys, belonging to the field of high-temperature alloy material processing and heat treatment technology. The stress-relief annealing method is as follows: First, the holding temperature T1 is determined based on the stress relaxation and controllable coarsening conditions of the γ′ phase in the nickel-iron-based superalloy; then, the holding time t1 and the first cooling rate V1 are determined according to the electrode diameter; finally, the recovery temperature T is determined from the holding temperature T1. m The holding time t1 determines the recovery time t2, and V1 determines the second cooling rate V2. The electrode is heated to T1 and held for t1, then controlled to cool down to T1 using V1. m The material is held at temperature t2, then cooled to the furnace outlet temperature at V2, and finally air-cooled to room temperature. This effectively solves the stress cracking problem of large-size nickel-iron-based high-temperature alloy VIM electrodes.

Description

Technical Field

[0001] This invention belongs to the field of high-temperature alloy material processing and heat treatment technology, specifically relating to a stress-relief annealing method for large-size nickel-iron-based high-temperature alloy induction electrodes. Background Technology

[0002] Vacuum induction melting (VIM) is a core process in the preparation of high-temperature alloy ingots. The induction electrodes cast in VIM serve as consumable electrodes for subsequent remelting processes, and their internal quality directly determines the stability of the subsequent melting process and the metallurgical quality of the final ingot. In recent years, with the surge in demand for large integrated forgings in fields such as nuclear power equipment, aero-engines, and gas turbines, high-temperature alloy ingots have continued to develop towards larger sizes. Producing large-sized ingots with diameters of 800mm or more and a single weight of 8-10 tons has become a key focus of the industry. Correspondingly, the specifications of VIM induction electrodes have also increased, with large-sized induction electrodes with diameters of 500-800mm and a single weight of 8-12 tons already entering the mass application stage.

[0003] However, large-scale VIM electrodes present significant technical challenges, including high stress and susceptibility to cracking. During the solidification and cooling stage after VIM casting, the large electrode size results in significant differences in cooling conditions between the core and edges, with instantaneous temperature differences exceeding 1000°C. This leads to thermal expansion strains as high as 1.2% to 1.5%. The superposition of this thermal stress and solidification stress easily exceeds the alloy's high-temperature fracture strength, causing macroscopic cracks to initiate and propagate within the electrode. Once the electrode cracks, it alters the effective conductive cross-sectional area, leading to severe melting rate fluctuations during subsequent vacuum arc remelting, and consequently, metallurgical quality accidents such as compositional segregation. In severe cases, it can even cause the entire electrode to fracture, triggering safety incidents. Therefore, effectively eliminating the internal stress of large-scale VIM electrodes has become a major technical bottleneck restricting the release of high-end high-temperature alloy production capacity and quality stability.

[0004] To address the aforementioned cracking problem, traditionally, the mature experience with small-to-medium-sized VIM electrodes (diameter ≤ 450mm) has been referenced, employing a simple thermal cycle process of "heating-holding-continuous cooling" for stress-relief annealing. However, directly applying this traditional process to large-sized electrodes has three fundamental drawbacks: First, uncontrolled radial temperature difference during the cooling stage leads to stress regeneration. The thermal diffusivity of large-sized electrodes is only 1 / 3 to 1 / 4 that of small-to-medium-sized electrodes. Taking a 550mm diameter VIM electrode as an example, even with an extremely slow continuous cooling rate of 8℃ / h, the peak temperature difference between the core and the surface is still as high as 160~190℃, and residual stress is regenerated due to the radial temperature difference after elimination. Traditional processes lack an understanding of when the temperature difference peak occurs, making it impossible to set up a heat-soaking pause at the point of maximum temperature difference to actively eliminate secondary stress. Second, the holding period is not designed to account for the differences in electrode diameter and alloy properties. The holding time often follows experience with small and medium-sized electrodes, without adjusting it according to the diameter, resulting in insufficient driving force for core stress relaxation. The holding temperature is only aimed at stress relaxation, ignoring the dissolution and coarsening behavior of the γ′ phase in nickel-iron-based superalloys. If this occurs in sensitive temperature ranges, it will cause hereditary deterioration of the microstructure, damaging the intrinsic properties of the electrode and interfering with subsequent melting. Finally, the cooling rate setting does not establish a quantitative correlation with the electrode diameter. Traditional processes use a single fixed cooling rate throughout, without distinguishing the differences in thermal response across different temperature zones. The core-surface temperature response of large-sized electrodes varies with temperature range and diameter; a fixed cooling rate inevitably amplifies the temperature difference in critical temperature zones, creating new stress peaks.

[0005] In summary, the traditional simple thermal cycle process of "heating-holding-continuous cooling" cannot effectively solve the stress cracking problem of large-size nickel-iron-based superalloy VIM electrodes due to its fundamental deficiencies in temperature difference control during cooling, differentiated design of holding parameters, and quantitative cooling rate. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the present invention aims to provide a stress-relieving annealing method for large-size induction electrodes of nickel-iron-based high-temperature alloys. This method can identify and utilize the temperature difference evolution law during the cooling process, optimize the stress relaxation effect through segmented temperature control and homogenization reheating, and quantitatively match the holding temperature with the γ′ phase transformation behavior, holding time and cooling rate with the electrode diameter.

[0007] To achieve the above objectives, the present invention employs the following technical solution: This invention provides a stress-relief annealing method for large-size nickel-iron-based high-temperature alloy induction electrodes, comprising the following steps: The holding temperature T1 is determined based on the stress relaxation conditions and controllable coarsening conditions of the γ′ phase in nickel-iron-based superalloys. The holding time t1 is determined based on the diameter D of the large-size induction electrode. The induction electrode is heated to the holding temperature T1 and then held for a holding time t1 at the holding temperature T1. Determine the recovery temperature T based on the insulation temperature T1. m The recovery time t2 is determined based on the heat preservation time t1, and the first cooling rate V1 is determined according to the diameter D of the large-size induction electrode. The induction electrode that has completed the heat preservation treatment is then subjected to controllable cooling treatment at the first cooling rate V1 to the recovery temperature T. m Then at the rewarming temperature T m The temperature is then adjusted to a warm-up time t2 for a homogenization treatment. Based on the first cooling rate V1, the second cooling rate V2 is determined. The induction electrode that has completed the heat recovery and homogenization treatment is cooled to the furnace exit temperature at the second cooling rate V2, and then removed from the furnace and air-cooled to room temperature.

[0008] In one embodiment, the holding temperature T1 is the intersection range of the stress relaxation condition and the controllable coarsening condition of the γ′ phase in the nickel-iron-based superalloy. The stress relaxation condition is the temperature range within which effective stress relaxation occurs in nickel-iron-based superalloys. The controllable coarsening condition of the γ′ phase is: the γ′ phase transformation temperature T of the nickel-iron-based superalloy. 相变 Above 15℃.

[0009] In one embodiment, the effective stress relaxation temperature range of the nickel-iron-based superalloy is 840~980°C; The γ′ phase transformation temperature T of the nickel-iron-based superalloy 相变 The temperature range is 820~880℃; The insulation temperature T1 is 840~895℃.

[0010] In one embodiment, the formula for calculating the heat preservation time t1 based on the diameter D of the large-size induction electrode is as follows: t1 = t0 + (D - 500) / 50 × 2; Where t0 is the basic heat preservation time, which is 6~12h; D is the diameter of the large-size induction electrode, in mm.

[0011] In one embodiment, the rewarming temperature T m It is 74% ± 20℃ of the insulation temperature T1.

[0012] In one embodiment, the reheating time t2 is equal to the heat preservation time t1.

[0013] In one embodiment, the first cooling rate V1 is inversely proportional to the diameter D of the large-size induction electrode, and the calculation formula is as follows: V1 = (10~12) × (500 / D)℃ / h; Where D is the diameter of the large-size induction electrode, in mm.

[0014] In one embodiment, the second cooling rate V2 is 2.2 to 2.8 times the first cooling rate V1.

[0015] In one embodiment, the furnace exit temperature is not higher than 350°C.

[0016] In one embodiment, the diameter D of the large-size induction electrode is ≥500mm.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention addresses three fundamental defects in the traditional "heating-holding-continuous cooling" process for stress-relief annealing of large-size nickel-iron-based superalloy VIM electrodes: uncontrolled cooling temperature difference, inconsistent holding parameters, and no quantitative correlation between cooling rates. It introduces a four-stage thermal cycle structure: holding (controllable coarsening of the γ′ phase), cooling, reheating, and secondary cooling. During the holding stage, it achieves synergistic treatment of residual stress relaxation and controllable coarsening of the γ′ phase, significantly improving the alloy's high-temperature plasticity and suppressing cracking tendency. During the reheating stage, it actively eliminates the cross-sectional temperature difference accumulated during the cooling process, suppressing regenerated thermal stress at its source. Ultimately, it achieves efficient and uniform removal of residual stress in large-size VIM electrodes and significantly improves the stability of the electrode in the next melting process (VAR). This enables precise control of residual stress and synergistic optimization of microstructure and properties, fundamentally solving the industry problem of stress cracking in large-size electrodes.

[0018] During the heat preservation stage, this invention overcomes the design limitations of traditional processes that only focus on stress relaxation. It establishes a dual constraint mechanism for heat preservation temperature, simultaneously addressing the requirements of sufficient residual stress relaxation and controllable coarsening of the γ′ phase. This ensures sufficient atomic diffusion driving force for the main relaxation of residual stress while reducing grain boundary stress concentration by controlling the appropriate coarsening of the γ′ phase, significantly improving the high-temperature plasticity of the alloy and suppressing cracking tendency at the intrinsic material level. It also avoids the hereditary deterioration of the microstructure caused by excessive dissolution or coarsening of the γ′ phase, thus guaranteeing subsequent melting performance. Addressing the problem that traditional processes do not consider electrode diameter differences and insufficient core stress relaxation during heat preservation time, this invention establishes a differentiated heat preservation time design method based on electrode diameter. This ensures that the core of large-diameter electrodes of different specifications achieves sufficient stress relaxation, solving the problem of residual core stress caused by directly transferring empirical parameters for small and medium-sized electrodes.

[0019] Addressing the core issues of uncontrolled radial temperature difference and secondary stress regeneration during continuous cooling in traditional processes, this invention precisely locates the peak node of the core-surface temperature difference during the cooling process of large-size electrodes through simulation calculations. Based on this, an innovative process of "controllable cooling to the peak temperature difference point and reheating for homogenization" is designed. At the node with the largest temperature difference, a homogenization stage is actively set to fully homogenize the temperature between the electrode core and the edge, eliminating the regenerated thermal stress caused by radial temperature difference from the source and completely solving the vicious cycle of "stress being eliminated and then regenerated" in traditional processes.

[0020] Regarding cooling rate design, this invention abandons the traditional, crude approach of using a single, fixed cooling rate throughout the entire process. Based on the differences in thermal response across different temperature zones of large-size electrodes, the cooling process is divided into two stages, each with a cooling rate design method quantitatively correlated with the electrode diameter: In the controllable cooling stage where the temperature difference accumulates rapidly, a cooling rate inversely proportional to the electrode diameter is used to precisely control the rate of temperature difference increase during this stage; in the secondary cooling stage where the temperature difference is smaller, a relatively faster cooling rate is used, significantly shortening the annealing cycle and improving production efficiency while ensuring stress control. This design method is based on regression analysis of a large amount of measured data, requiring no complex numerical simulation calculations and directly determining values ​​based on the electrode diameter. It possesses strong practicality and operability in the field, fully adaptable to the needs of industrial mass production.

[0021] Ultimately, this invention, through the synergistic effect of a four-stage thermal cycling structure and a fully quantitative parameter design, achieves efficient and uniform removal of residual stress in large-scale VIM electrodes. This effectively prevents the initiation and propagation of macroscopic cracks inside the electrodes, ensures the uniformity of the effective conductive cross-sectional area of ​​the electrodes, and significantly improves the melting rate stability of the subsequent vacuum consumable remelting process. It fundamentally eliminates metallurgical quality and safety accidents such as compositional segregation and electrode fracture caused by electrode cracking. This provides core technical support for the stable mass production of large-scale nickel-iron-based high-temperature alloy induction electrodes and powerfully promotes the release of production capacity and quality improvement of high-end high-temperature alloy large ingots. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the simulation calculation of the γ′ phase transition temperature of the present invention; Figure 2 This is a schematic diagram of the simulation calculation results of the temperature drop process of the ingot core and edge in an embodiment of the present invention; Figure 3 The diagram shows the growth of the γ′ phase in the products obtained in Examples 1 to 3 and Comparative Example 1 of this invention. Detailed Implementation

[0023] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0024] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0025] The present invention will now be described in further detail with reference to the accompanying drawings: A stress-relief annealing method for large-size nickel-iron-based superalloy induction electrodes, wherein the temperature-time curve of the stress-relief annealing method consists of the following four stages: (a) Insulation stage: The large-size induction electrode is heated to a holding time t1 and maintained at that temperature for a certain holding time t1.

[0026] (II) Controllable cooling stage: Cool to the recovery temperature T at the first cooling rate V1. m .

[0027] (III) Heating and warming stage: At the recovery temperature T m Then, maintain the temperature for a certain period of time t2.

[0028] (iv) Second cooling phase: The furnace temperature is reduced to the furnace exit temperature at a second cooling rate V2, and then air-cooled to room temperature after exiting the furnace.

[0029] Preferably, the temperature nodes and times for each stage are determined in the following manner: 1. Insulation stage: The insulation temperature T1 is determined based on the stress relaxation conditions and controllable coarsening conditions of the γ′ phase of nickel-iron-based superalloys, and the insulation time t1 is determined based on the diameter D of the large-size induction electrode.

[0030] Specifically, the holding temperature T1 needs to simultaneously meet the stress relaxation requirement and the controllable coarsening requirement of the γ′ phase. That is, the holding temperature T1 is the intersection range of the stress relaxation condition and the controllable coarsening condition of the γ′ phase in the nickel-iron-based superalloy.

[0031] (1) The stress relaxation condition is: Based on the empirical range determined by typical nickel-iron-based high-temperature alloys, it is 840~980℃.

[0032] (2) The controllable coarsening condition of the γ′ phase is: T γ′ =T 相变 +15℃ T 相变 T is the γ′ phase transformation temperature of nickel-iron-based superalloys. γ′ The temperature at which the γ′ phase can be controlled to coarsen is 1.

[0033] The γ′ phase transition temperature (T) was determined based on simulation calculations. 相变 The temperature range is 820~880℃, T γ′ The temperature ranges from 835 to 895℃. Figure 1 ) Based on the requirements of the two conditions, the insulation temperature T1 ranges from 840 to 895℃.

[0034] Specifically, the heat preservation time t1 is determined based on the diameter D of the large-size induction electrode.

[0035] The heat preservation time t1 (diameter correction) is obtained from the following empirical formula: t1 = t0 + (D - 500) / 50 × 2; In the formula: t0 is the basic heat preservation time, which is determined according to the traditional stress-relief annealing process and is taken as 6~12h; D is the diameter of the large-size induction electrode, in mm.

[0036] 2. Controlled cooling and warming stages: The warming temperature T is determined based on the holding temperature T1. m The reheating time t2 is determined based on the heat preservation time t1, and the first cooling rate V1 is determined based on the diameter D of the large-size induction electrode.

[0037] Specifically, the temperature T is the return temperature. m The endpoint temperature of the controllable cooling phase is constrained by the following conditions: T m =74%T1±20℃ T1 is the insulation temperature, calculated through simulation. When the temperature drops to 74% of the insulation temperature ( Figure 2 The temperature difference between the middle and edge of the ingot is greatest at this point, which is considered the optimal reheating temperature. An additional 20°C tolerance is added. The final reheating temperature is T. mThe temperature ranges from 598 to 682℃.

[0038] Specifically, the holding time during the warming phase is the same as the holding time during the heat preservation phase, that is, the warming time t2 is equal to the heat preservation time t1.

[0039] Preferably, the cooling rate for each stage is determined as follows: For large-size induction electrodes (diameter ≥ 500 mm), this invention adopts the following simplified empirical formula based on regression of a large amount of measured data, which can be directly obtained without complex simulation calculations.

[0040] Specifically, the first cooling rate V1 in the controllable cooling stage is inversely proportional to the diameter D of the large-size induction electrode, and the calculation formula is as follows: V1 = (10~12) × (500 / D)℃ / h; Where D is the diameter of the large-size induction electrode, in mm.

[0041] Recommended value range: D=500mm: V1=10~12℃ / h D=550mm: V1=9~11℃ / h D=600mm: V1=8~10℃ / h D=650mm: V1=7.5~9℃ / h D=700mm: V1=7~8.5℃ / h Based on a 500mm diameter induction electrode, the first cooling rate is set at 10~12℃ / h. For every 50mm increase in the diameter of the induction electrode, the range of the first cooling rate is adjusted down by 0.5~1℃ / h.

[0042] 3. Secondary cooling stage: The second cooling rate V2 is determined based on the first cooling rate V1.

[0043] Specifically, the second cooling rate V2 is 2.2 to 2.8 times that of the first cooling rate V1.

[0044] The formula for calculating the second cooling rate V2 is as follows: V2 = (2.2~2.8) × V1; V1 is the first cooling rate.

[0045] Preferably, the furnace exit temperature is ≤350℃, and the furnace exit temperature is air-cooled to room temperature after exiting the furnace.

[0046] All the following examples and comparative examples used GH4169 nickel-iron-based superalloy mother liquor prepared by the same vacuum induction melting (VIM) process to cast cylindrical as-cast induction electrodes of corresponding specifications. The individual electrode weights were approximately 6.5 tons for the 550mm specification, approximately 7.8 tons for the 600mm specification, and approximately 9.2 tons for the 650mm specification, with a length of 3200mm for all. After the electrode surface was machined on a lathe to remove oxide scale and casting flash, it was processed in the same 15-ton trolley-type argon-protected annealing furnace. The furnace temperature uniformity was controlled within ±5℃, and argon gas with a purity of ≥99.999% was introduced throughout the process. The furnace pressure was maintained at a slightly positive pressure of 0.02~0.05MPa.

[0047] All samples were tested using the same standards and methods: Residual stress detection: X-ray diffraction was used to determine the residual stress on the surface (2 mm from the surface), and the blind hole method was used to determine the residual stress at 1 / 2 radius and the core. Five points were tested at each location and the average value was taken. Tissue observation: Samples were taken from the surface, half radius, and core of the electrode head, middle, and tail. After grinding and polishing, the morphology of the γ′ phase was observed using a scanning electron microscope (SEM), and the average size and distribution uniformity were statistically analyzed. Internal defect detection: 100% full-volume flaw detection is performed using a water immersion ultrasonic flaw detector, with a detection sensitivity ≥ φ2mm flat-bottomed holes; Subsequent melting verification: All electrodes were melted in the same 10-ton vacuum arc remelting (VAR) furnace, using the same arc initiation, arc stabilization and capping processes, and the melting rate, current, voltage and vacuum data were automatically recorded throughout the process.

[0048] Example 1 The stress-relief annealing process used for a 550mm induction electrode consists of a temperature-time curve comprising the following four stages: (a) Insulation stage The induction electrode was heated to 850°C and maintained for 12 hours.

[0049] (II) Controllable cooling stage Cool down to the recovery temperature of 650℃ at a cooling rate of 10℃ / h.

[0050] (III) Heating and warming stage Maintain at the warm temperature for 12 hours.

[0051] (iv) Second cooling phase The temperature was reduced to 350°C at a cooling rate of 25°C / h, and then air-cooled to room temperature after being removed from the furnace.

[0052] Samples were taken after annealing to observe the phase composition. Subsequent smelting continued, and the melting rate remained stable throughout the smelting process.

[0053] Test and verification results: Residual stress: 254 MPa at the surface, 172 MPa at half the radius, and 89 MPa at the core. The overall stress distribution is uniform with no local stress concentration. Tissue observation: The γ′ phase is uniformly spherical, with an average size of 120 nm and a volume fraction of about 18%. There is no abnormal growth, dissolution or local aggregation. The tissue consistency in different locations is good. Flaw detection results: 100% full-volume flaw detection revealed no internal cracks, pores, or inclusions. VAR melting: The melting current is stable at 8.7kA, the voltage is 24.2V, the average melting rate is 6.2kg / min, and the melting rate fluctuation range is controlled within ±0.3kg / min. There are no arc breaks, short circuits or electrode vibrations throughout the process. The final ingot surface is smooth, without cold shuts or shrinkage cavities, and the compositional segregation is ≤5%, meeting the requirements for aerospace-grade forgings.

[0054] Example 2 The stress-relief annealing process used for a 600mm induction electrode consists of a temperature-time curve comprising the following four stages: (a) Insulation stage The induction electrode was heated to 850°C and maintained for 12 hours.

[0055] (II) Controllable cooling stage Cool down to the recovery temperature of 650℃ at a cooling rate of 9℃ / h.

[0056] (III) Heating and warming stage Maintain at the warm temperature for 12 hours.

[0057] (iv) Second cooling phase The temperature was reduced to 350°C at a cooling rate of 23°C / h, and then air-cooled to room temperature after being removed from the furnace.

[0058] Samples were taken after annealing to observe the phase composition. Subsequent smelting continued, and the melting rate remained stable throughout the smelting process.

[0059] Test and verification results: Residual stress: 247MPa at the surface, 154MPa at half the radius, 63MPa at the core, stress gradient ≤50MPa / 100mm; Tissue observation: The γ′ phase is uniformly spherically distributed with an average size of 127 nm and a volume fraction of approximately 17%, with no tissue abnormalities. Flaw detection results: No internal defects; VAR melting: The melting current was stable at 8.9kA, the voltage at 24.3V, the average melting rate was 6.25kg / min, and the melting rate fluctuation range was ±0.28kg / min; the final ingot passed the flaw detection test and had good compositional uniformity.

[0060] Example 3 The stress-relief annealing process used for a 650mm induction electrode consists of a temperature-time curve comprising the following four stages: (a) Insulation stage The induction electrode was heated to 850°C and maintained for 16 hours.

[0061] (II) Controllable cooling stage Cool down to the recovery temperature of 650℃ at a cooling rate of 8℃ / h.

[0062] (III) Heating and warming stage Maintain at the warm temperature for 16 hours.

[0063] (iv) Second cooling phase Cool to 350°C at a cooling rate of 20°C / h, then air-cool to room temperature after removal from the furnace.

[0064] Samples were taken after annealing to observe the phase composition. Subsequent smelting continued, and the melting rate remained stable throughout the smelting process.

[0065] Supplementary test and verification results: Residual stress: 241 MPa at the surface, 162 MPa at half the radius, and 65 MPa at the core; there are no obvious stress concentration areas. Tissue observation: The γ′ phase is uniformly spherically distributed with an average size of 112 nm and a volume fraction of approximately 18%. The tissue difference between different locations is ≤4%. Flaw detection results: No internal defects; VAR melting: The melting current is stabilized at 9.0kA, the voltage is 24V, the average melting rate is 6.2kg / min, and the melting rate fluctuation range is ±0.2kg / min; the final ingot quality meets the requirements for billets used in industrial gas turbines.

[0066] Comparative Example 1 The stress-relief annealing process used for a 600mm induction electrode consists of a temperature-time curve comprising the following two stages: (a) Insulation stage The induction electrode was heated to 850°C and maintained for 4 hours.

[0067] (II) Cooling Phase The temperature was reduced to 350°C at a cooling rate of 25°C / h, and then air-cooled to room temperature after being removed from the furnace.

[0068] Samples were taken after annealing to observe the phase composition. Subsequent smelting continued, and the melting rate fluctuated during the smelting process.

[0069] Test and verification results: Residual stress: 391 MPa at the surface, 284 MPa at half the radius, and as high as 75 MPa in the core, with a stress gradient exceeding 85.4 MPa / 100 mm, indicating severe stress concentration in the core; Tissue observation: The size distribution of the γ′ phase is extremely uneven, with an average size of 54 nm. Small γ′ phase aggregates exist in local areas. The difference between the core tissue and the surface tissue exceeds 12%, indicating obvious tissue heritability. Flaw detection results: An axial microcrack with a length of 148 mm was found at the middle half radius and the core of the electrode. There was no obvious plastic deformation at the crack tip. VAR melting: The current fluctuation reached ±0.9kA in the early stage of melting, with an average melting rate of 2.5kg / min, but the melting rate fluctuation range was as high as ±8.8kg / min. There were 4 arc interruptions throughout the process, which forced multiple manual interventions to stabilize the arc. Finally, there were multiple cold shut defects on the surface of the ingot. Internal flaw detection revealed a porosity defect larger than φ17mm at the edge of the ingot, and the composition segregation reached 24%, which could not meet the requirements for subsequent forging and was scrapped.

[0070] like Figure 3 As shown, the microstructure of Comparative Example 1 is finer, and the microstructure stress generated by the precipitation is greater, which leads to instability in the melting process.

[0071] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode, characterized in that, Includes the following steps: The holding temperature T1 is determined based on the stress relaxation conditions and controllable coarsening conditions of the γ′ phase in nickel-iron-based superalloys. The holding time t1 is determined based on the diameter D of the large-size induction electrode. The induction electrode is heated to the holding temperature T1 and then held for a holding time t1 at the holding temperature T1. Determine the recovery temperature T based on the insulation temperature T1. m The recovery time t2 is determined based on the heat preservation time t1, and the first cooling rate V1 is determined according to the diameter D of the large-size induction electrode. The induction electrode that has completed the heat preservation treatment is then subjected to controllable cooling treatment at the first cooling rate V1 to the recovery temperature T. m Then at the rewarming temperature T m The temperature is then adjusted to a warm-up time t2 for a homogenization treatment. Based on the first cooling rate V1, the second cooling rate V2 is determined. The induction electrode that has completed the heat recovery and homogenization treatment is cooled to the furnace exit temperature at the second cooling rate V2, and then removed from the furnace and air-cooled to room temperature.

2. The stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode according to claim 1, characterized in that, The heat preservation temperature T1 is the intersection range of the stress relaxation condition and the controllable coarsening condition of the γ′ phase in the nickel-iron-based superalloy. The stress relaxation condition is the temperature range within which effective stress relaxation occurs in nickel-iron-based superalloys. The controllable coarsening condition of the γ′ phase is: the γ′ phase transformation temperature T of the nickel-iron-based superalloy. 相变 Above 15℃.

3. The stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode according to claim 2, characterized in that, The effective stress relaxation temperature range of the nickel-iron-based superalloy is 840~980℃; The γ′ phase transformation temperature T of the nickel-iron-based superalloy 相变 The temperature range is 820~880℃; The insulation temperature T1 is 840~895℃.

4. The stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode according to claim 1, characterized in that, The formula for calculating the heat preservation time t1 based on the diameter D of the large-size induction electrode is as follows: t1 = t0 + (D - 500) / 50 × 2; Where t0 is the basic heat preservation time, which is 6~12h; D is the diameter of the large-size induction electrode, in mm.

5. The stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode according to claim 1, characterized in that, The rewarming temperature T m It is 74% ± 20℃ of the insulation temperature T1.

6. The stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode according to claim 1, characterized in that, The reheating time t2 is equal to the heat preservation time t1.

7. The stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode according to claim 1, characterized in that, The first cooling rate V1 is inversely proportional to the diameter D of the large-size induction electrode, and the calculation formula is as follows: V1 = (10~12) × (500 / D)℃ / h; Where D is the diameter of the large-size induction electrode, in mm.

8. The stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode according to claim 1, characterized in that, The second cooling rate V2 is 2.2 to 2.8 times the first cooling rate V1.

9. The stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode according to claim 1, characterized in that, The furnace exit temperature shall not exceed 350℃.

10. The stress-relief annealing method for a large-size nickel-iron-based high-temperature alloy induction electrode according to claim 1, characterized in that, The diameter D of the large-size induction electrode is ≥500mm.

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