Methods for improving the uniformity of microstructure and preventing cracking in GH4738 alloy billets during billet preparation

Abstract

This invention provides a billet control method for improving the uniformity of microstructure and preventing cracking in GH4738 alloy billets, belonging to the field of high-temperature alloy hot working technology. The method first constructs a hot deformation model of GH4738 alloy, a physical parameter model, and a rheological stress model based on Simufact software. Boundary conditions are set according to the actual production environment of the enterprise, and process conditions are set according to the actual capacity and operating conditions of the enterprise's equipment. Then, billet process parameters are set according to the billet process. A JMAK recrystallization kinetic model is constructed, and finally, a Normalized Cockcroft & Latham model is constructed. Elongation at different temperatures is obtained through high-temperature tensile experiments, and a critical damage factor for GH4738 alloy is constructed. This method calculates and iteratively obtains alloy billets with uniform microstructure and no cracking by adjusting and optimizing process parameters.

Description

Technical Field

[0001] This invention relates to the field of high-temperature alloy hot working technology, and in particular to a billet control method for improving the uniformity of the microstructure of GH4738 alloy billets and preventing cracking. Background Technology

[0002] Billet preparation involves continuously hot-deforming a homogenized ingot to induce recrystallization of the coarse as-cast structure, resulting in a finer-grained structure. This transforms the ingot into a forged billet for further processing. The microstructure after billet preparation directly affects the quality of the final product. The multi-pass, multi-heating process of billet preparation involves complex temperature, deformation, and stress conditions in the ingot. In practice, production often relies on experience-based process design and operation, making it difficult to comprehensively consider details and provide timely feedback on the billet preparation results. Chinese patent application CN 114592115 A proposes a deformation process for coarse-grained or mixed-grained GH4738 alloy bars, but it is not applicable to billet preparation for large ingot-shaped discs. Chinese patent application CN 110747418 B proposes a homogenization and billet preparation method, achieving uniform grains, but it has limitations in complex working conditions and specific dimensional requirements. Chinese patent application CN 110747419 A proposes a production process for high-quality GH4738 alloy, but its description of the billet preparation stage is not detailed enough, and its control over details is insufficient. In summary, there is currently a lack of universally applicable process control guidelines for the billet opening process of the GH4738 alloy system.

[0003] High-temperature alloys have a small hot working window and relatively poor plasticity, often resulting in cracking during the billet preparation process. Cracks need to be removed by grinding to prevent through-cracks. This not only increases material waste but also adds to the processing steps, raising production costs and reducing efficiency. The causes of cracking are related to many factors such as temperature, stress, and strain, making them difficult to assess in actual production. Research on cracking issues during the billet preparation of GH4738 alloy is scarce.

[0004] With the development of numerical computing, people have gained theoretical support by establishing relevant organizational models through industrial finite element software, which has promoted the development of process control. However, controlling the deformation of billets through multiple firings and passes also poses challenges to the accuracy of the models and the rationality of the simulation process. Summary of the Invention

[0005] This invention provides a billet control method to improve the uniformity of the microstructure of GH4738 alloy billets and prevent cracking. It uses the JMAK recrystallization kinetic microstructure evolution model to predict the grain size change of the billet during the deformation process, and the Normalized Cockcroft & Latham cracking damage model to predict the surface and internal cracking of the billet. A calculation process is designed to calculate the microstructure and cracking tendency of GH4738 alloy during the billet opening process.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] A method for improving the uniformity of microstructure and preventing cracking in GH4738 alloy billets during billet preparation includes the following steps:

[0008] S1. Construct geometric models of GH4738 alloy billet, robotic arm, and high-speed forging machine based on Simufact software, and mesh them;

[0009] S2. Input the physical parameters and rheological stress of GH4738 alloy into the Simufact software to construct a material property model;

[0010] S3. Based on the actual production environment of the enterprise, set the thermal boundary conditions in Simufact software, such as the heat transfer coefficient between the billet and air, and the heat transfer coefficient between the billet and the anvil.

[0011] S4. Based on the actual capacity and working conditions of the enterprise's equipment, set the enterprise's process conditions in the Simufact software, such as the equipment parameters of the high-speed forging machine model, the equipment parameters of the robot model, and the range of worker operation time.

[0012] S5. Set the process parameters for Simufact software to be calculated according to the billet opening process;

[0013] S6. Construct models of tissue evolution, including dynamic recrystallization, subdynamic recrystallization, and grain growth models;

[0014] S7. Construct a cracking damage model. Using the Normalized Cockcroft & Latham model, obtain the elongation of GH4738 alloy at different temperatures through high-temperature tensile tests, and construct the critical cracking factor C of GH4738 alloy at different temperatures. f The change curve and the cracking criterion formula of the cracking damage model are as follows:

[0015]

[0016] In the formula, P is the cracking criterion, and σ * The maximum tensile stress is σ, and the equivalent stress is σ. For equivalent strain, εf For equivalent crack strain, C f (T) is the critical cracking factor at temperature T. When P≥1, the material is considered to have localized cracking, and when P<1, the material is considered safe.

[0017] Where, σ * , σ, ε f C f (T) were obtained by calculating the stress and strain of the billet during the billet deformation process using Simufact software.

[0018] S8. Combining the microstructure evolution model established in step S6 and the cracking damage model established in step S7, the Simufact solver is used to calculate the billet opening process to obtain the billet opening process, microstructure evolution and cracking damage results. The calculation results show the actual production situation. Problems such as difficulty in production, failure to meet microstructure requirements and billet cracking caused by improper processes are predicted. Iterative calculations are performed by repeatedly modifying the process and simulating to determine the billet opening control process parameters.

[0019] Specifically, step S1 involves:

[0020] A three-dimensional geometric model of the high-speed forging machine, alloy billet, and robot arm was created using CAD design software according to the actual production dimensions. This model was then imported into Simufact and divided into hexahedral meshes (the hexahedral mesh element size is 10-50mm).

[0021] The physical parameters of the GH4738 alloy in step S2 include thermal conductivity and heat capacity.

[0022] The process conditions in step S4 include the maximum downward pressure of the upper anvil, the empty stroke downward pressure speed, the pressurization rate and the return speed; the feed speed, return speed, rotation speed, clamping depth and clamping force of the robot arm; the transfer time for entering and exiting the furnace; and the time, method, material and size of the cladding.

[0023] The billet opening process parameters in step S5 include the cold billet heating regime during the heating process, the reheating regime between heating cycles, and the transfer time between exiting and entering the furnace; the upsetting rate and pressing amount during the upsetting process; and the feeding amount, pressing amount, feed regime, and rotation angle for each pass during the drawing process.

[0024] The tissue evolution model in step S6 is as follows:

[0025] Zener-Hollomen parameters:

[0026]

[0027] Peak strain model:

[0028]

[0029] Percentage of dynamic recrystallization:

[0030]

[0031] 50% strain during recrystallization:

[0032]

[0033] Dynamic recrystallization grain size:

[0034] d dyn =109Z -0.04

[0035] Subdynamic recrystallization fraction:

[0036]

[0037] Subdynamic recrystallization occurs at 50% of its time:

[0038]

[0039] Subdynamic recrystallization grain size:

[0040]

[0041] Grain growth model:

[0042]

[0043] In the formula: Z is the Zener-Hollomen parameter; The equivalent rate of change; d is the peak strain; d0 is the initial grain size; X dyn This represents the volume percentage of dynamic recrystallization. For equivalent change; The strain at which 50% dynamic recrystallization occurs; d dyn For dynamic recrystallization grain size; X mdyn The volume percentage of subdynamic recrystallization; t 0.5 The time required for 50% subdynamic recrystallization to occur; t m The time for subdynamic recrystallization to occur is given by d, where t is the time for grain growth; mdyn d represents the subdynamic recrystallized grain size; d represents the grain size after growth; R and T represent the gas constant and absolute temperature, respectively.

[0044] The above technical solution has at least the following advantages compared with the existing technology:

[0045] The above scheme developed a JMAK microstructure evolution model and a Normalized C&L cracking damage model for GH4738 alloy. The predicted microstructure evolution model and cracking damage model were coupled into the finite element software (Simufact) to optimize the upsetting and drawing process with multiple passes and heat treatments.

[0046] The microstructure evolution model includes a dynamic recrystallization model, a subdynamic recrystallization model, and a grain growth model. It can calculate the average experienced size, average grain size, recrystallization fraction, recrystallized grain size, and other related results during the upsetting and drawing process, and can complete the task of predicting and analyzing the grain structure during the upsetting and drawing process.

[0047] The cracking damage model can predict the damage and cracking caused by billet deformation and temperature drop throughout the entire upsetting and drawing process.

[0048] By constructing a GH4738 alloy billet model, the production results of billeting according to the process parameters of the billet process sheet can be predicted. By adjusting and optimizing the process parameters, calculations can be performed iteratively to obtain alloy billets with uniform structure and no cracking. Attached Figure Description

[0049] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 This is a process flow diagram of a billet control method for improving the uniformity of microstructure and preventing cracking of GH4738 alloy billet according to the present invention.

[0051] Figure 2 The results are shown in Example 1 of this invention after the first upsetting process, where (a) is a grain size level diagram, (b) is the cracking criterion P, and (c) is a temperature field distribution diagram.

[0052] Figure 3 The results are shown in Example 1 of this invention after the first elongation. (a) is a grain size level diagram, (b) is the cracking criterion P, and (c) is a temperature field distribution diagram.

[0053] Figure 4 The results are shown in Example 1 of this invention after the second elongation. (a) is a grain size level diagram, (b) is the cracking criterion P, and (c) is a temperature field distribution diagram.

[0054] Figure 5The results are shown in Example 1 of this invention after the third upsetting process, where (a) is a grain size level diagram, (b) is the cracking criterion P, and (c) is a temperature field distribution diagram.

[0055] Figure 6 The results are shown in Example 2 of this invention after the first upsetting process, where (a) is a grain size level diagram, (b) is the cracking criterion P, and (c) is a temperature field distribution diagram.

[0056] Figure 7 The results after the first elongation in Example 2 of the present invention are shown in (a) the grain size level diagram, (b) the cracking criterion P, and (c) the temperature field distribution diagram.

[0057] Figure 8 The results are shown in Example 2 of this invention after the second upsetting process, where (a) is a grain size level diagram, (b) is the cracking criterion P, and (c) is a temperature field distribution diagram.

[0058] Figure 9 The results are shown in Example 2 of this invention after the second elongation. (a) is a grain size level diagram, (b) is the cracking criterion P, and (c) is a temperature field distribution diagram. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0060] This invention provides a method for controlling the billet opening process to improve the uniformity of the microstructure of GH4738 alloy billets and prevent cracking.

[0061] like Figure 1 As shown, the method includes the following steps:

[0062] S1. Construct geometric models of GH4738 alloy billet, robotic arm, and high-speed forging machine based on Simufact software, and mesh them;

[0063] S2. Input the physical parameters and rheological stress of GH4738 alloy into the Simufact software to construct a material property model;

[0064] S3. Based on the actual production environment of the enterprise, set the heat transfer coefficient between the billet and air, and the heat transfer coefficient between the billet and the anvil in the Simufact software, and set the thermal boundary conditions.

[0065] S4. Based on the actual capacity and working conditions of the enterprise's equipment, set the equipment parameters of the high-speed forging machine model, the equipment parameters of the robot model, and the enterprise's process conditions for the worker's operation time range in the Simufact software.

[0066] S5. Set the process parameters for Simufact software to be calculated according to the billet opening process;

[0067] S6. Construct models of tissue evolution, including dynamic recrystallization, subdynamic recrystallization, and grain growth models;

[0068] S7. Construct a cracking damage model using the Normalized Cockcroft & Latham model. Obtain the elongation of GH4738 alloy at different temperatures through high-temperature tensile tests, and construct the critical damage factor C of GH4738 alloy at different temperatures. f The change curve and the cracking criterion formula of the cracking damage model are as follows:

[0069]

[0070] In the formula, P is the cracking criterion; when P ≥ 1, the material is considered to have localized cracking; when P < 1, the material is considered safe. σ * The maximum tensile stress is σ, and the equivalent stress is σ. For equivalent strain, ε f For equivalent crack strain, C f (T) is the critical cracking factor at temperature T.

[0071] S8. Combining the microstructure evolution model established in step S6 and the cracking damage model established in step S7, the Simufact solver is used to calculate the billet opening process to obtain the billet opening process, microstructure evolution and cracking damage results. The calculation results show the actual production situation. Problems such as difficulty in production, failure to meet microstructure requirements and billet cracking caused by improper processes are predicted. Iterative calculations are performed by repeatedly modifying the process and simulating to determine the billet opening control process parameters.

[0072] The following description, in conjunction with specific embodiments, illustrates this point.

[0073] Example 1

[0074] Calculations were performed based on the 4t GH4738 alloy billet forging process. Simufact software was used to construct geometric models of the GH4738 alloy billet, the robotic arm, and the high-speed forging machine. The billet size was φ580mm*1740mm, and a mesh was created in the software with a unit size of 40mm. The thermal conductivity, heat capacity, and rheological stress of the GH4738 alloy were input, and thermal boundary conditions such as the heat transfer coefficient between the billet and air, and the heat transfer coefficient between the billet and the anvil were set in the software. The upsetting reduction rate was set to 15mm / s and the drawing reduction rate to 60mm / s according to the company's high-speed forging machine settings. Based on the process sheet in Table 1, a five-pass billet forging process was set, including the deformation amount for each pass in each pass. Calculations were performed using a software solver that coupled the microstructure evolution model and the cracking model.

[0075] Table 1 GH4738 Alloy Billet Process Sheet

[0076]

[0077] After the first upsetting, the main grain size of the billet was controlled to around grade 3 or finer, but the upper surface of the ingot head and the area near the clamp handle were dead zones where the grain size remained unchanged. The first upsetting did not cause significant cracking or damage. Figure 2 As shown.

[0078] After the first upsetting, the billet undergoes transfer in the furnace, heating, and further transfer. Following drawing and octagonal shaping, the grain size in most areas of the billet, except for the clamp area, is around grade 3-4. Based on the deformation process, it can be determined that the core has not undergone complete recrystallization. The upper surface of the ingot head and the area near the clamp are dead zones, where the grain size remains unchanged. According to the cracking criterion P of the longitudinal section of the billet after the first drawing, the core damage cracking criterion P value is relatively large, but does not exceed 1. Figure 3 Based on the temperature distribution after drawing, it can be found that after drawing for two 2 hours at 1170℃, the surface temperature of the billet drops to about 1000℃, but the core temperature of the billet is still greater than 1160℃. Therefore, attention should be paid to the core temperature rise during the drawing process.

[0079] After the second upsetting, the main grain size of the billet was controlled to around level 5-6, resulting in more complete recrystallization compared to the first upsetting, and a reduction in the dead zone at the ingot head. The second upsetting did not cause significant cracking damage. The billet surface temperature dropped to around 1040℃.

[0080] After the second upsetting, the billet, after being transferred back to the furnace, heated, and then transferred again, undergoes drawing and octagonal shaping. Except for the handle area, the grain size in most areas of the billet is around grade 3-4, coarser than the grains from the previous upsetting. This indicates that the grains grew rapidly during the reflow process, and the drawing deformation was insufficient to refine the grains further. Figure 4The cracking criterion P of the longitudinal section of the billet after the second drawing was still relatively high, and the location of the cracking was similar to that after the first drawing, indicating that the damage sensitivity is related to the intersection of the anvil feed during free forging and the end forging method. According to the temperature distribution after drawing, it can be found that after drawing for two 2 hours in the furnace at 1170℃, the surface temperature of the billet dropped to about 960℃, but the core temperature of the billet was still greater than 1160℃.

[0081] After the third upsetting, the main grain size of the billet was controlled at around level 5-6, which was similar to the result of the second upsetting. The dead zone of the ingot head was further reduced, and no obvious cracking or damage was caused. After the third fire upsetting, the overall temperature of the billet was uniform, and the central temperature rise was not obvious, which met the expected results. Figure 5 .

[0082] Example 2

[0083] Calculations were performed based on the 2.5t GH4738 alloy billet forging process. Simufact software was used to construct geometric models of the GH4738 alloy billet, the robotic arm, and the high-speed forging machine. The billet size was φ460mm*1600mm, and a mesh was created in the software with a unit size of 40mm. The thermal conductivity, heat capacity, and rheological stress of the GH4738 alloy were input, and thermal boundary conditions such as the heat transfer coefficient between the billet and air, and the heat transfer coefficient between the billet and the anvil were set in the software. The upsetting reduction rate was set to 20mm / s and the drawing reduction rate to 40mm / s according to the company's high-speed forging machine settings. Based on the process sheet in Table 2, a four-pass billet forging process was set, including the deformation amount for each pass in each pass. The calculations were performed using a software solver that coupled the microstructure evolution model and the cracking model. The GH4738 alloy billet forging process sheet is shown in Table 2 below.

[0084] Table 2 GH4738 Alloy Billet Process Sheet

[0085]

[0086] In the first batch, excluding the dead zone at the ingot head, the grain size of the billet body is above grade 2, and can reach grade 4-7 at the ingot center. After deformation, the ingot body temperature is between 950.05-1175.91℃. P is the plastic damage criterion; a value greater than 1 may indicate cracking. Therefore, it can be concluded that no cracking occurred in the ingot body. Figure 6 As shown.

[0087] In the first extraction, excluding the dead zone at the ingot head, the grain size on the ingot surface was between grade 4 and 6, and the grain size at the ingot core was between grade 2 and 4. After deformation, the ingot body temperature was between 944.80 and 1158.47℃. No cracks occurred at the center of the ingot body, but there was a risk of cracking in some areas of the surface, especially at the edge of the anvil where pressure was applied. Figure 7 .

[0088] In the second forging, excluding the dead zone at the ingot head, the grain size of the billet body is above level 2, and the grain size at the ingot center can reach level 4-7. After deformation, the temperature of the ingot body is between 1103.27-1172.45℃. The forging billet did not crack, but cracks may have occurred at the junction of the clamp and the billet.

[0089] In the second pass, excluding the dead zone at the ingot head, the grain distribution in the ingot body was uniform, all between grades 2 and 5. After deformation, the ingot body temperature ranged from 946.09 to 1160.05℃. No cracks occurred in the center of the ingot body, but there was a risk of cracking in some areas of the surface, especially at the pressing point on the anvil edge. Figure 9 .

[0090] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. The scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for controlling the billet opening process to improve the uniformity of microstructure and prevent cracking in GH4738 alloy billets, characterized in that, The steps include the following: S1. Construct geometric models of GH4738 alloy billet, robotic arm, and high-speed forging machine based on Simufact software, and mesh them; S2. Input the physical parameters and rheological stress of GH4738 alloy into the Simufact software to construct a material property model; S3. Based on the actual production environment of the enterprise, set the heat transfer coefficient between the billet and air, and the heat transfer coefficient between the billet and the anvil in the Simufact software, and set the thermal boundary conditions. S4. Based on the actual capacity and working conditions of the enterprise's equipment, set the equipment parameters of the high-speed forging machine model, the equipment parameters of the robot model, and the enterprise's process conditions for the worker's operation time range in the Simufact software. S5. Set the process parameters for Simufact software to be calculated according to the billet opening process; S6. Construct models of tissue evolution, including dynamic recrystallization, subdynamic recrystallization, and grain growth models; S7. Construct a cracking damage model. Using the Normalized Cockcroft & Latham model, obtain the elongation of GH4738 alloy at different temperatures through high-temperature tensile tests, and construct the critical cracking factor C of GH4738 alloy at different temperatures. f The change curve and the cracking criterion formula of the cracking damage model are as follows: In the formula, P is the cracking criterion, and σ * The maximum tensile stress is σ, and the equivalent stress is σ. For equivalent strain, ε f For equivalent crack strain, C f (T) is the critical cracking factor at temperature T; when P≥1, local cracking occurs in the material, and when P<1, the material is safe. S8. Combining the microstructure evolution model established in step S6 and the cracking damage model established in step S7, the Simufact solver is used to calculate the billet opening process to obtain the billet opening process, microstructure evolution and cracking damage results. The calculation results show the actual production situation. Problems such as difficulty in production, failure to meet microstructure requirements and billet cracking caused by improper processes are predicted. Iterative calculations are performed by repeatedly modifying the process and simulating to determine the billet opening control process parameters.

2. The cogging control method for improving the microstructure uniformity and preventing cracking of GH4738 alloy blank according to claim 1, characterized in that, Specifically, step S1 involves: A three-dimensional geometric model of the forging machine, alloy billet, and robot arm was created using CAD design software based on the actual dimensions of the production process. This model was then imported into Simufact and meshed with hexahedrons.

3. The cogging control method for improving the microstructure uniformity and preventing cracking of GH4738 alloy blank according to claim 1, characterized in that, The physical parameters of the GH4738 alloy in step S2 include thermal conductivity and heat capacity.

4. The cogging control method for improving the microstructure uniformity and preventing cracking of GH4738 alloy blank according to claim 1, characterized in that, The process conditions in step S4 include the maximum downward pressure of the upper anvil, the empty stroke downward pressure speed, the pressurization rate and the return speed; the feed speed, return speed, rotation speed, clamping depth and clamping force of the robot arm; the transfer time for entering and exiting the furnace; and the time, method, material and size of the cladding.

5. The cogging control method for improving the microstructure uniformity and preventing cracking of GH4738 alloy blank according to claim 1, characterized in that, The billet opening process parameters in step S5 include the cold billet heating regime during the heating process, the reheating regime between heating cycles, and the transfer time between exiting and entering the furnace; the upsetting rate and pressing amount during the upsetting process; and the feeding amount, pressing amount, feed regime, and rotation angle for each pass during the drawing process.

6. The cogging control method for improving the microstructure uniformity and preventing cracking of GH4738 alloy blank according to claim 1, characterized in that, The tissue evolution model in step S6 is as follows: Zener-Hollomen parameters: Peak strain model: Percentage of dynamic recrystallization: 50% strain during recrystallization: Dynamic recrystallization grain size: d dyn = 109Z -0.04 Subdynamic recrystallization fraction: Subdynamic recrystallization occurs at 50% of its time: Subdynamic recrystallization grain size: Grain growth model: In the formula: Z is the Zener-Hollomen parameter; The equivalent rate of change; d is the peak strain; d0 is the initial grain size; X dyn This represents the volume percentage of dynamic recrystallization. For equivalent change; The strain at which 50% dynamic recrystallization occurs; d dyn For dynamic recrystallization grain size; X mdyn The volume percentage of subdynamic recrystallization; t 0.5 The time required for 50% subdynamic recrystallization to occur; t m The time for subdynamic recrystallization to occur is given by d, where t is the time for grain growth; mdyn d represents the subdynamic recrystallized grain size; d represents the grain size after growth; R and T represent the gas constant and absolute temperature, respectively.

7. The billet control method for improving the uniformity of microstructure and preventing cracking of GH4738 alloy billet according to claim 2, characterized in that, The size of the hexahedral mesh unit is 10-50mm.

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