High-performance alloy steel water-air alternate quenching process design and optimization method

Abstract

The invention discloses a high-performance alloy steel water-air alternate quenching process design and optimization method, and relates to the technical field of heat treatment. The method comprises the following steps: step 1, preparing a cylindrical sample, and testing a cooling curve after quenching; step 2, establishing a finite element model of the cylindrical sample, performing initial calibration on the model by utilizing an actually measured cooling curve, and inverting an initial heat exchange coefficient; 3, preparing a conical sample, quenching, and testing the hardness and the structure; 4, establishing a finite element model of the conical sample, fitting an actually measured hardness gradient by utilizing a simulated tissue hardness relation, and reversely solving an equivalent heat exchange coefficient; 5, updating boundary conditions of the finite element model of the large-size workpiece, and dynamically correcting the model; and 6, finite element simulation is conducted on different quenching processes, and the optimal quenching process is determined. According to the method, the water-air alternate quenching process of the high-performance alloy steel large-section component can be systematically designed and optimized, and the residual stress and the cracking risk are stably controlled while the hardenability and the mechanical property are guaranteed.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology, and in particular to a method for designing and optimizing a molten-air alternating quenching process for high-performance alloy steel. Background Technology

[0002] High-performance alloy steels (such as 42CrMo and 45 steel) are widely used in large load-bearing components such as ship crankshafts, steam turbine rotors, main shafts of construction machinery, and wind turbine main shafts. These components are typically large in size and thick in cross-section, requiring high hardenability, strength, and service reliability. In engineering, a quenching and tempering process involving rapid cooling after austenitization is commonly used to achieve high strength and toughness. Water quenching, due to its strong cooling capacity, low cost, and relatively environmental friendliness, is one of the main methods for achieving high hardenability in thick-section alloy steels. However, for large-section components, direct water quenching results in a large temperature difference between the surface and the core, leading to uneven phase transformation and easily generating significant thermal and structural stresses, resulting in quenching cracks and severe deformation. To reduce these risks, oil quenching or polymer media are often used in production to reduce the cooling rate, or parameters such as stirring and residence time are manually adjusted during water quenching. However, it is often difficult to simultaneously achieve high hardenability, crack resistance, and environmental protection requirements.

[0003] To achieve a balance between performance and safety, the industry has proposed the water-air alternating time-controlled quenching (ATQ) approach. This involves alternating stages of water cooling, air cooling, and re-water cooling after austenitization. This maintains a high cooling rate during the high-temperature stage to ensure thorough quenching, while air cooling during the low-temperature stage mitigates the temperature gradient and phase transformation stress, thereby improving residual stress and reducing the risk of cracking. In current practical applications, process parameters such as the time and number of cycles for each stage of the water-air alternating process largely rely on experience and repeated experimentation. This results in long development cycles, sensitivity to equipment and media conditions, and a lack of a unified, quantifiable design methodology.

[0004] On the other hand, finite element quenching simulation can simultaneously calculate the temperature field, microstructure field, and residual stress field. If realistic and reliable boundary heat transfer conditions can be obtained, it is possible to predict the temperature evolution, microstructure distribution, and stress level under different water-air alternating processes in a virtual environment, which can be used for process scheme selection and optimization. However, in actual production, the spraying state, medium contamination, and aging of quenching equipment will cause the cooling capacity to change over time, and the heat transfer coefficient calibrated in a single test will quickly become distorted. The finite element model cannot reflect the actual cooling capacity of the equipment in a timely manner.

[0005] Current national standards have already provided methods for evaluating the cooling capacity of quenching cooling media using cylindrical and conical specimens. In particular, conical specimens can conveniently reflect the overall cooling capacity through hardness gradients, facilitating regular on-site testing in the workshop. However, existing standards and engineering practices basically only use conical specimens as an evaluation tool for "whether the medium is good or not," without further converting the hardness gradient results of conical specimens into heat transfer parameters that can be directly used for finite element simulations, and without establishing a complete technical route of "conical specimen evaluation → finite element model correction → water-air alternating process design." Summary of the Invention

[0006] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is how to determine the water-air alternating quenching parameters, while taking into account hardenability, residual stress and crack resistance, and the calibration of the heat transfer coefficient of the finite element model can be updated in a timely manner as the equipment cooling capacity changes.

[0007] To achieve the above objectives, this invention provides a method for designing and optimizing a high-performance alloy steel molten-air alternating quenching process, comprising the following steps: Step 1: Prepare cylindrical specimens, quench them, and then test the cooling curves. Step 2: Establish a finite element model of the cylindrical sample, perform initial calibration of the model using the measured cooling curve, and invert the initial heat transfer coefficient. Step 3: Prepare a conical sample, quench it, and then test its hardness and microstructure; Step 4: Establish a finite element model of the conical specimen, fit the measured hardness gradient using the simulated microstructure hardness relationship, and inversely calculate the equivalent heat transfer coefficient. Step 5: Update the boundary conditions of the finite element model of the large workpiece and dynamically correct the model; Step 6: Perform finite element simulations on different quenching processes to determine the optimal quenching process.

[0008] In some embodiments, in step one, the cylindrical sample has a diameter of 100 mm and a length of 300 mm. The cylindrical sample is first austenitized at 850°C for 1 hour and then air-cooled. Then, the cylindrical sample is heated to 850°C and held for 1 hour. After that, it is pre-cooled in air for 200 seconds and finally quickly immersed in room water for full immersion cooling.

[0009] In some embodiments, in step one, the K-type thermocouples are embedded inside the cylindrical sample at the following locations: 5 mm below the surface of the cylindrical sample at the center, 0.5R at the mean diameter, and along the geometric center axis.

[0010] In some embodiments, in step two, the size of the cylindrical specimen finite element model is the same as the size of the cylindrical specimen. The cylindrical specimen finite element model is an axisymmetric thermo-structural coupling model. A convective heat transfer boundary is applied to the outer surface of the cylindrical specimen finite element model. The heat transfer coefficient varies with temperature zones, namely 300-500℃ and 500-800℃. The initial heat transfer coefficient is inverted by comparing the simulated cooling curve with the measured cooling curve.

[0011] In some embodiments, in step three, the material of the conical specimen is the same as that of the cylindrical specimen. The diameter of the large end of the conical specimen is 30 mm, the diameter of the small end is 15 mm, and the total length is 120 mm. The conical specimen is first austenitized at 850℃ for 1 h and then air-cooled. Then the conical specimen is heated to 850℃ and held for 1 h, and then quenched according to the actual process of the equipment to be evaluated.

[0012] In some embodiments, in step three, the sampling positions for hardness testing of the conical specimen are one point every 5 mm along the axial direction, with a total of 15 to 20 hardness testing points arranged from the large end to the small end. The microstructure observation positions of the conical specimen are the large end, middle part, and small end cross sections of the conical specimen.

[0013] In some implementations, in step four, the finite element model of the conical specimen has the same dimensions as the conical specimen. The finite element model of the conical specimen is an axisymmetric model. The heating and quenching time regime consistent with the test is applied in the finite element model of the conical specimen. The convective heat transfer coefficient function of the temperature zone is set on the outer surface of the finite element model of the conical specimen. The heat transfer coefficient of each temperature zone is used as the parameter to be calibrated. The axial simulated hardness curve is calculated by phase transformation and microstructure-hardness relationship, and compared with the measured hardness curve to inversely determine the equivalent heat transfer coefficient of the current equipment.

[0014] In some implementations, in step five, the finite element model of the large workpiece is a thermal-structural-organic coupled finite element model. The equivalent heat transfer coefficient obtained in steps two and four is used as a characterization parameter of the actual cooling capacity of the quenching equipment to be evaluated under the current water medium and operating conditions, and is introduced into the finite element model of the large workpiece.

[0015] In some implementations, in step six, different quenching process simulations are performed by simulating temperature, microstructure, and residual stress for different combinations of water cooling time, air cooling time, and number of cycles. The different combinations of quenching process parameters are: pre-cooling time 150-250s, water cooling time 300-500s, air cooling time 60-120s, and re-water cooling time 120-300s. The evaluation indicators are the surface and core hardness of the large workpiece, the depth of the hardened layer, and the maximum residual tensile stress.

[0016] The above-mentioned design and optimization method for the molten-air alternating quenching process of high-performance alloy steel is applied to large-section load-bearing components.

[0017] Compared with the prior art, the present invention has the following advantages: (1) It can quickly calibrate and continuously update the quenching boundary conditions of small samples, screen the water-air alternating process in advance in the virtual environment, significantly reduce the number of quenching trials for large workpieces, reduce the risk of cracking and deformation, and make the process match the actual cooling capacity of the equipment for a long time, which is convenient for engineering promotion.

[0018] (2) There is no need to arrange a large number of thermocouples to measure the cooling curve. The equivalent heat transfer parameters can be obtained quickly through conventional hardness testing, which simplifies the heat transfer coefficient inversion and model calibration process. The evaluation results of the conical sample can be directly used to update the finite element boundary conditions, thus opening up the link of "cooling capacity detection - numerical simulation - process design".

[0019] (3) It can simultaneously take into account hardenability and residual stress control, provide a variety of simulated and verified water-air alternating process schemes for different safety margins and service requirements, reduce experience trial and error, improve the pertinence and flexibility of process design, and can quickly regenerate new schemes when the equipment cooling capacity changes.

[0020] The following will further explain the concept, specific structure, and technical effects of the present invention in conjunction with the accompanying drawings, so as to fully understand the purpose, features, and effects of the present invention. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the method steps in an embodiment of the present invention; Figure 2 This is a schematic diagram of the geometry of the cylindrical sample and the arrangement of thermocouples in an embodiment of the present invention; Figure 3 These are the measured results of cooling curves at different radius positions of the cylindrical sample in the embodiments of the present invention; Figure 4 These are simulation results of cooling curves at different radius positions of the cylindrical sample in an embodiment of the present invention; Figure 5 This is a schematic diagram of the geometry and dimensions of the conical sample according to an embodiment of the present invention; Figure 6 This is the axial hardness gradient curve of the conical sample in an embodiment of the present invention; Figure 7 The microstructure of the conical sample at different locations in an embodiment of the present invention; Figure 8 This is a radial distribution diagram of the residual stress in the cylindrical specimen according to an embodiment of the present invention. Detailed Implementation

[0022] The preferred embodiments of the present invention are described below with reference to the accompanying drawings to make the technical content clearer and easier to understand. The present invention can be embodied in many different forms, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0023] In the accompanying drawings, components with the same structure are indicated by the same numerical designation, and components with similar structures or functions are indicated by similar numerical designations. The dimensions and thicknesses of each component shown in the drawings are arbitrary, and the present invention does not limit the dimensions and thicknesses of each component. To make the illustrations clearer, the thickness of some components has been appropriately exaggerated in the drawings.

[0024] Addressing the problems of existing heat treatment technologies for high-performance alloy steels: Water-air alternating quenching parameters are mainly determined through experience and trial and error, making it difficult to simultaneously consider hardenability, residual stress, and crack resistance. The heat transfer coefficients in finite element models are often calibrated only once and cannot be updated in a timely manner according to changes in equipment cooling capacity. The hardness gradient of conical specimens is currently mainly used to classify cooling capacity levels, but it cannot establish a quantitative correspondence with specific heat transfer coefficients in the finite element model, making it difficult to directly use for boundary condition calibration. Optimization of the water-air alternating quenching process often focuses on hardness or hardened layer as a single objective, lacking a comprehensive design that simultaneously considers the magnitude and uniformity of residual stress distribution, making it difficult to provide systematic process solutions for different service conditions. This invention proposes a design and optimization method for water-air alternating quenching process of high-performance alloy steel based on finite element simulation and evaluation of the cooling capacity of conical specimens. This method introduces finite element inversion and process optimization on the basis of existing evaluation methods for the cooling capacity of conical specimens. The hardness distribution of conical specimens is used to dynamically calibrate the quenching boundary conditions. With the updated finite element model as the core, the water-air alternating quenching process of large cross-section components of high-performance alloy steel is systematically designed and optimized, thereby ensuring hardenability and mechanical properties while stably controlling residual stress and cracking risk.

[0025] like Figure 1 The diagram shows the steps of the design and optimization method for the high-performance alloy steel molten-air alternating quenching process of the present invention. The method includes the following steps: Step 1: Prepare cylindrical specimens, quench them, and then test the cooling curves. Step 2: Establish a finite element model of the cylindrical sample, perform initial calibration of the model using the measured cooling curve, and invert the initial heat transfer coefficient. Step 3: Prepare a conical sample, quench it, and then test its hardness and microstructure; Step 4: Establish a finite element model of the conical specimen, fit the measured hardness gradient using the simulated microstructure hardness relationship, and inversely calculate the equivalent heat transfer coefficient. Step 5: Update the boundary conditions of the finite element model of the large workpiece and dynamically correct the model; Step 6: Perform finite element simulations on different quenching processes to determine the optimal quenching process.

[0026] In some embodiments, in step one, the cylindrical sample has a diameter of 100 mm and a length of 300 mm. The cylindrical sample is first austenitized at 850°C for 1 hour and then air-cooled. Then, the cylindrical sample is heated to 850°C and held for 1 hour. After that, it is pre-cooled in air for 200 seconds and finally quickly immersed in room water for full immersion cooling.

[0027] In some embodiments, in step one, the K-type thermocouples are embedded inside the cylindrical sample at the following locations: 5 mm below the surface of the cylindrical sample at the center, 0.5R at the mean diameter, and along the geometric center axis.

[0028] In some embodiments, in step two, the size of the cylindrical specimen finite element model is the same as the size of the cylindrical specimen. The cylindrical specimen finite element model is an axisymmetric thermal-structural coupling model. A convective heat transfer boundary is applied to the outer surface of the cylindrical specimen finite element model. The heat transfer coefficient varies with temperature zones, which are 300-500℃ and 500-800℃. The initial heat transfer coefficient is inverted by comparing the simulated cooling curve with the measured cooling curve.

[0029] In some embodiments, in step three, the material of the conical specimen is the same as that of the cylindrical specimen. The diameter of the large end of the conical specimen is 30 mm, the diameter of the small end is 15 mm, and the total length is 120 mm. The conical specimen is first austenitized at 850℃ for 1 h and then air-cooled. Then the conical specimen is heated to 850℃ and held for 1 h, and then quenched according to the actual process of the equipment to be evaluated.

[0030] In some embodiments, in step three, the sampling positions for hardness testing of the conical specimen are one point every 5 mm along the axial direction, with a total of 15 to 20 hardness testing points arranged from the large end to the small end. The microstructure observation positions of the conical specimen are the cross sections of the large end, middle, and small end of the conical specimen.

[0031] In some implementations, in step four, the finite element model of the conical specimen has the same dimensions as the conical specimen. The finite element model of the conical specimen is an axisymmetric model. The heating and quenching time regime consistent with the test is applied in the finite element model of the conical specimen. The convective heat transfer coefficient function of the temperature zone is set on the outer surface of the finite element model of the conical specimen. The heat transfer coefficient of each temperature zone is used as the parameter to be calibrated. The axial simulated hardness curve is calculated by phase transformation and microstructure-hardness relationship, and compared with the measured hardness curve to inversely determine the equivalent heat transfer coefficient of the current equipment.

[0032] In some implementations, in step five, the finite element model of the large workpiece is a thermal-structural-organic coupled finite element model. The equivalent heat transfer coefficient obtained in steps two and four is used as a characterization parameter of the actual cooling capacity of the quenching equipment to be evaluated under the current water medium and operating conditions, and is introduced into the finite element model of the large workpiece.

[0033] In some implementations, in step six, different quenching process simulations are performed by simulating temperature, microstructure, and residual stress for different combinations of water cooling time, air cooling time, and number of cycles. The different combinations of quenching process parameters are: pre-cooling time 150-250s, water cooling time 300-500s, air cooling time 60-120s, and re-water cooling time 120-300s. The evaluation indicators are the surface and core hardness of the large workpiece, the depth of the hardened layer, and the maximum residual tensile stress.

[0034] The aforementioned high-performance alloy steels include 42CrMo and 45 steel.

[0035] The finite element modeling described above uses general-purpose finite element software such as Abaqus and Ansys.

[0036] The above-mentioned design and optimization method for the molten-air alternating quenching process of high-performance alloy steel is applied to large load-bearing components such as ship crankshafts, steam turbine rotors, engineering machinery main shafts, and wind turbine main shafts.

[0037] The present invention will use an axisymmetric component made of 42CrMo high-performance alloy steel as an example to describe the specific implementation and realization of the design and optimization method of alternating quenching process for high-performance alloy steel by molten steel and air.

[0038] Step 1: Prepare cylindrical specimens, quench them, and then test the cooling curves.

[0039] High-performance alloy steel bars of grade 42CrMo were selected, with a mass percentage composition of 0.42C-0.9Cr-0.2Mo-0.7Mn and the balance being Fe. Cylindrical specimens (hereinafter referred to as cylindrical specimens) with a diameter of 100 mm and a length of 300 mm were prepared by machining. After austenitization at 850℃ for 1 h, they were air-cooled to obtain a uniform initial microstructure.

[0040] like Figure 2 The diagram shows a cylindrical specimen and thermocouple arrangement. Holes were drilled axially at the center (5 mm below the surface), 0.5R (the mean diameter), and the geometric center of the specimen to embed type K thermocouples. The thermocouple leads were connected to a temperature acquisition instrument. The specimen was heated to 850℃ and held for 1 hour, then removed and pre-cooled in air for 200 seconds. It was then rapidly immersed in room water for full immersion cooling, and the temperature-time curves at each measuring point were recorded. Figure 3 The figure shows the measured results of cooling curves at different radius positions.

[0041] Step 2: Establish a finite element model of the cylindrical sample, perform initial calibration of the model using measured cooling curves, and invert the initial heat transfer coefficient.

[0042] A finite element model of a cylindrical specimen was established in a general finite element software. The initial heat transfer coefficient was inverted by comparing the simulated cooling curve with the measured cooling curve.

[0043] Specifically, a finite element axisymmetric thermo-structural coupled model of a cylindrical specimen with a diameter of 100 mm and a length of 300 mm was established. Axisymmetric elements were divided, and temperature-dependent thermal conductivity, specific heat, density, and latent heat of phase transformation parameters of 42CrMo steel were input. Existing CCT curves and a phase transformation kinetic model were used to describe the transformation from austenite to pearlite, bainite, and martensite. A convective heat transfer boundary was applied to the outer surface of the model specimen. Assuming that the heat transfer coefficient varies with temperature zones, the heat transfer coefficient of each temperature zone was used as a parameter to be determined. By adjusting the heat transfer coefficient of each temperature zone, the cooling curves of each measurement point obtained from the simulation (e.g., ...) were adjusted. Figure 4 (as shown) and measured curves (as shown) Figure 3 As shown, the error was controlled within a predetermined range in key temperature zones such as 800~500℃ and 500~300℃, and the initial heat transfer coefficient-temperature relationship of the equipment and water medium was obtained.

[0044] Step 3: Prepare a conical specimen, quench it, and then test its hardness and microstructure.

[0045] A high-performance alloy steel bar of grade 42CrMo was selected and a conical specimen was prepared by machining. The large end diameter was 30 mm, the small end diameter was 15 mm, and the total length was 120 mm. The shape and dimensions of the conical specimen are as follows. Figure 5 As shown, the conical sample was austenitized at 850℃ for 1 hour and then air-cooled to obtain a uniform initial microstructure.

[0046] The conical sample was reheated to 850℃ and held for 1 hour. After pre-cooling for 200 seconds, it was fully immersed in room temperature water for cooling. After quenching, approximately 15-20 hardness test points were arranged along the axial direction at 5 mm intervals, from the large end to the small end. The "axial position-hardness" distribution curve was obtained as shown below. Figure 6 As shown in the figure. Simultaneously, samples were taken from the large end, middle, and small end sections of the conical specimen to observe its microstructure type and distribution, obtaining the microstructure as shown in the figure. Figure 7 As shown.

[0047] Step 4: Establish a finite element model of the conical specimen, fit the measured hardness gradient using the simulated microstructure hardness relationship, and inversely calculate the equivalent heat transfer coefficient.

[0048] A finite element model of a conical specimen is established in a general finite element software. The heat transfer coefficient parameter to be calibrated is set in the convection boundary. The axial simulated hardness curve is calculated by phase transformation and microstructure-hardness relationship, and compared with the measured hardness curve to inversely determine the equivalent heat transfer coefficient or cooling capacity index of the current equipment.

[0049] Specifically, a finite element axisymmetric model of a conical specimen with dimensions identical to the actual conical specimen is established. The temperature-related thermophysical properties, phase transformation kinetics, and microstructure-hardness correspondence of the alloy steel are input. Heating and quenching time regimes consistent with those in the experiment are applied to the model. Convective heat transfer coefficient functions for temperature zones are set on the outer surface of the conical specimen model, with the heat transfer coefficients of each temperature zone serving as parameters to be calibrated. The temperature field and microstructure field during the quenching process are solved, the microstructure composition at each axial position is calculated, and converted into simulated hardness, yielding the "axial position – simulated hardness" curve.

[0050] The hardness curve of the conical sample obtained by simulation is compared with the measured hardness curve (e.g.) Figure 6 The simulated hardness gradient was compared with the measured hardness gradient (as shown in the figure), and the error between the two was used as the target. The convective heat transfer coefficient of each temperature zone was adjusted and the calculation was repeated until the simulated hardness gradient basically matched the measured hardness gradient. At the same time, the simulated microstructure distribution and the microstructure characteristics of the measured sample were compared (e.g., Figure 7 As shown in the figure, the matching degree is verified. Finally, the equivalent "heat transfer coefficient-temperature" relationship between the current equipment and the water medium is obtained, which is used as a quantitative characterization parameter of the cooling capacity of the equipment. This parameter is then used to update the boundary conditions in the subsequent finite element model of large-size workpieces for high-performance alloy steel quenching, so as to realize the periodic calibration and process correction of changes in cooling capacity.

[0051] Step 5: Update the boundary conditions of the finite element model of the large workpiece and dynamically correct the model.

[0052] Based on the equivalent "heat transfer coefficient-temperature" relationship obtained by inversion of the cylindrical and conical specimens, it is used as a characterization parameter of the actual cooling capacity of the quenching equipment to be evaluated under the current water medium and operating conditions, and introduced into the finite element model of the large workpiece (hereinafter referred to as the large workpiece).

[0053] Specifically, segmented convective heat transfer boundary conditions, consistent with those in the calibration test, are applied to the outer surface of the large workpiece's thermo-structural-microstructure coupled finite element model. The heat transfer coefficient's variation with temperature is given by the functional relationship determined in steps two and four, thus avoiding the use of empirical constants or idealized heat transfer coefficient assumptions. This approach ensures that the thermal boundary conditions in the large workpiece model accurately reflect the actual equipment's cooling capacity, providing a reliable boundary basis for subsequent simulation analysis of different combinations of quenching process parameters.

[0054] Step 6: Perform finite element simulation optimization on different quenching processes to determine the optimal quenching process.

[0055] Based on the dynamically corrected (calibrated) updated model, water cooling time, air cooling time, and number of cycles are used as design variables. Temperature, microstructure, and residual stress are simulated for different combinations of water cooling, air cooling time, and number of cycles. Surface and core hardness, hardened layer depth, and maximum residual tensile stress are statistically analyzed. A multi-objective optimization algorithm is used to select the set of approximately optimal process parameters under different performance trade-offs to meet the hardness, hardened layer depth, and stress requirements of large workpieces.

[0056] Specifically, in the calibrated model, a series of water-air alternating process parameter combinations were set, such as pre-cooling time 150–250 s, water cooling time 300–500 s, air cooling time 60–120 s, and re-water cooling time 120–300 s. The quenching process was simulated and optimized for different parameter combinations, outputting the martensite volume fraction and residual stress distribution in the radial direction of the quenched sample. By comparing the surface hardness, hardened layer depth, and maximum residual tensile stress of each scheme, a set of water-air alternating quenching process parameters that balances hardenability and crack resistance was selected. Figure 8 The figure shows the radial residual stress distribution of the cylindrical specimen under this scheme.

[0057] This invention presents a design and optimization method for high-performance alloy steel water-air alternating quenching processes based on finite element simulation and conical specimen cooling capacity evaluation. This method enables rapid calibration and continuous updating of quenching boundary conditions using small samples. It allows for pre-screening of water-air alternating processes in a virtual environment, significantly reducing the number of quenching trials for large workpieces, lowering the risk of cracking and deformation, and ensuring the process maintains long-term alignment with the actual cooling capacity of the equipment, facilitating engineering implementation. It eliminates the need for numerous thermocouple measurements of cooling curves; equivalent heat transfer parameters can be quickly obtained through conventional hardness testing, simplifying the heat transfer coefficient inversion and model calibration process. The evaluation results from conical specimens can be directly used to update finite element boundary conditions, establishing a seamless link between "cooling capacity detection – numerical simulation – process design." It can simultaneously consider hardenability and residual stress control, providing multiple simulation-verified water-air alternating process schemes for different safety margins and service requirements. This reduces trial-and-error, improves the targeting and flexibility of process design, and allows for rapid regeneration of new schemes when the equipment's cooling capacity changes.

[0058] The high-performance alloy steel molten-air alternating quenching process design method proposed in this invention does not require large-scale modification of existing quenching equipment. It mainly relies on general-purpose finite element analysis software and phase transformation / constitutive material models, cylindrical and conical specimens prepared according to national standards, conventional temperature acquisition systems and hardness / microstructure detection equipment, and corresponding data processing and process design procedures.

[0059] The high-performance alloy steel molten-air alternating quenching process design method proposed in this invention has significant engineering practicality and industrial application prospects.

[0060] (1) Technological advantages and performance improvement potential

[0061] It can quickly obtain suitable water-air alternating quenching process parameters for key components such as shafts and discs of different sizes and alloy systems; it considers the coupling of temperature field, microstructure field and residual stress field in the process design stage, which can effectively control the risk of quenching cracks and deformation while ensuring strength and hardness requirements; using the quantitative calibration technology of cooling capacity of conical specimens, the process design model can be updated in real time according to the changes in equipment cooling capacity, thereby improving the quality consistency in long-term production.

[0062] (2) Feasibility of production implementation

[0063] The sample design and evaluation methods are compatible with the existing national standard system, making them easy for factory technicians to understand and use. The finite element simulation and optimization modules can be packaged into process design software or digital twin systems, and can be integrated with existing process management systems and production management systems. The project implementation cost is significantly lower than the cost of repeated testing of large specimens, and the investment recovery period is short, making it suitable for promotion in large forging steel plants, heavy machinery enterprises, wind power and shipbuilding enterprises.

[0064] (3) Transformation prospects and application scenarios

[0065] It can be commercialized as "high-performance alloy steel heat treatment process design software" or "quenching cooling capacity online monitoring and process optimization system" to provide technical services and software products for enterprises. When applied in high-reliability scenarios such as wind power main shafts, nuclear power pump casings, and marine engineering equipment, it is expected to significantly reduce the scrap rate and rework costs caused by quenching cracks, and has significant economic and social benefits.

[0066] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for designing and optimizing a high-performance alloy steel molten-air alternating quenching process, characterized in that, Includes the following steps: Step 1: Prepare cylindrical specimens, quench them, and then test the cooling curves. Step 2: Establish a finite element model of the cylindrical sample, perform initial calibration of the model using the measured cooling curve, and invert the initial heat transfer coefficient. Step 3: Prepare a conical sample, quench it, and then test its hardness and microstructure; Step 4: Establish a finite element model of the conical specimen, fit the measured hardness gradient using the simulated microstructure hardness relationship, and inversely calculate the equivalent heat transfer coefficient. Step 5: Update the boundary conditions of the finite element model of the large workpiece and dynamically correct the model; Step 6: Perform finite element simulations on different quenching processes to determine the optimal quenching process.

2. The method for designing and optimizing the molten-air alternating quenching process of high-performance alloy steel as described in claim 1, characterized in that, In step one, the cylindrical sample has a diameter of 100 mm and a length of 300 mm. The cylindrical sample is first austenitized at 850℃ for 1 h and then air-cooled. Then the cylindrical sample is heated to 850℃ and held for 1 h. After that, it is pre-cooled in air for 200 s and finally quickly placed in room water for full immersion cooling.

3. The method for designing and optimizing the molten-air alternating quenching process of high-performance alloy steel as described in claim 2, characterized in that, The K-type thermocouples are embedded inside the cylindrical sample at the following locations: 5 mm below the surface of the cylindrical sample at the center, 0.5R at the mid-diameter, and along the axial direction of the geometric center.

4. The method for designing and optimizing the molten-air alternating quenching process of high-performance alloy steel as described in claim 3, characterized in that, In step two, the dimensions of the cylindrical specimen finite element model are the same as those of the cylindrical specimen. The cylindrical specimen finite element model is an axisymmetric thermo-structural coupling model. A convective heat transfer boundary is applied to the outer surface of the cylindrical specimen finite element model. The heat transfer coefficient varies with temperature zones, namely 300–500℃ and 500–800℃. The initial heat transfer coefficient is inverted by comparing the simulated cooling curve with the measured cooling curve.

5. The method for designing and optimizing the molten-air alternating quenching process of high-performance alloy steel as described in claim 1, characterized in that, In step three, the material of the conical sample is the same as that of the cylindrical sample. The conical sample has a large end diameter of 30 mm, a small end diameter of 15 mm, and a total length of 120 mm. The conical sample is first austenitized at 850℃ for 1 hour and then air-cooled. Then, the conical sample is heated to 850℃ and held for 1 hour, and then quenched according to the actual process of the equipment to be evaluated.

6. The method for designing and optimizing the molten-air alternating quenching process of high-performance alloy steel as described in claim 5, characterized in that, The sampling locations for the hardness test of the conical specimen are one point every 5 mm along the axial direction, with a total of 15 to 20 hardness test points arranged from the large end to the small end. The locations for observing the microstructure of the conical specimen are the cross-sections of the large end, middle, and small end of the conical specimen.

7. The method for designing and optimizing the molten-air alternating quenching process of high-performance alloy steel as described in claim 6, characterized in that, In step four, the dimensions of the finite element model of the conical specimen are the same as those of the conical specimen. The finite element model of the conical specimen is an axisymmetric model. The heating and quenching time regime consistent with the test is applied to the finite element model of the conical specimen. The convective heat transfer coefficient function of the temperature zone is set on the outer surface of the finite element model of the conical specimen. The heat transfer coefficient of each temperature zone is used as the parameter to be calibrated. The axial simulated hardness curve is calculated by phase transformation and microstructure-hardness relationship, and compared with the measured hardness curve to inversely determine the equivalent heat transfer coefficient of the current equipment.

8. The method for designing and optimizing the molten-air alternating quenching process of high-performance alloy steel as described in claim 1, characterized in that, In step five, the finite element model of the large workpiece is a thermal-structural-organic coupled finite element model. The equivalent heat transfer coefficient obtained in steps two and four is used as a characterization parameter of the actual cooling capacity of the quenching equipment to be evaluated under the current water medium and operating conditions, and is introduced into the finite element model of the large workpiece.

9. The method for designing and optimizing the molten-air alternating quenching process of high-performance alloy steel as described in claim 1, characterized in that, In step six, the simulation of different quenching processes involves simulating temperature, microstructure, and residual stress for different combinations of water cooling time, air cooling time, and number of cycles. The different combinations of quenching process parameters are: pre-cooling time 150–250 s, water cooling time 300–500 s, air cooling time 60–120 s, and re-water cooling time 120–300 s. The evaluation indicators are the surface and core hardness of the large workpiece, the depth of the hardened layer, and the maximum residual tensile stress.

10. The method for designing and optimizing the molten-air alternating quenching process for high-performance alloy steel as described in any one of claims 1-9, characterized in that, The method is applied to large-section load-bearing components.

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