Method for improving the shape control process of carburizing and quenching heat treatment by using heat treatment numerical value

Abstract

The present invention discloses a method for improving the shape control process of carburizing and quenching heat treatment by using heat treatment numerical values, establishing a gear finite element model, setting the temperature heat transfer boundary conditions and carburizing diffusion boundary conditions of the model surface in the carburizing and quenching heat treatment; solving the thermophysical parameters of the material, using thermal simulation tests and heat transfer coefficient determination tests to respectively solve the phase change plasticity coefficients of A, B, and M and the convection heat transfer coefficient of the quenching oil; establishing the mixed rules, heat transfer, diffusion, phase change dynamics, stress-strain constitutive and regression equations of carburizing and quenching heat treatment, and writing a numerical simulation program; numerically simulating the heat of multi-process carburizing and quenching to obtain the distribution laws of temperature field, diffusion field, organization field, and stress-strain field; and then determining the optimal heat treatment process. The present invention realizes the multi-field coupling of carburizing and quenching heat treatment, can improve the consistency between simulation and experimental results, and obtains the influence of process parameters on shape control such as distortion and residual stress.

Description

Technical Field

[0001] The invention relates to a carburizing and quenching heat treatment process simulation technology for low-carbon alloy steel materials, and specifically to a method for improving a carburizing and quenching heat treatment shape control process by utilizing heat treatment numerical values. Background Art

[0002] The carburizing and quenching heat treatment process is often used in the manufacture of gears, which are key components of mechanical systems. It can improve the fatigue resistance, corrosion resistance and wear resistance of the gear surface, and improve the mechanical properties and fatigue strength of the material. Carburizing is to add carbon to the surface of low-carbon steel within the austenitizing temperature range, generally between 850℃ and 950℃, and quench it after a period of heat preservation, so that the high-carbon surface layer of the gear forms martensite and completes the hardening. In this way, a high-carbon martensitic shell with good wear resistance and fatigue resistance is superimposed on a tough low-carbon alloy steel core. Despite these advantages, the gears of key components will be deformed after carburizing and quenching heat treatment. The obvious deformation of key components will lead to adverse effects. For example, the deformation of the gears after carburizing and quenching will increase the noise of the transmission system. In order to ensure improved performance while controlling deformation, it is very important to choose a suitable carburizing and quenching process. With the development of computer numerical simulation technology, the optimal heat treatment process is determined by simulating and analyzing different heat treatment processes, and the optimal heat treatment process is experimentally verified. The influence of carburizing and quenching heat treatment process parameters on the interaction between temperature field, diffusion field, stress / deformation field and microstructure changes is further analyzed. With the purpose of multi-objective optimization, a high-performance, low-deformation heat treatment process is determined.

[0003] Kong Lingli et al. used finite element simulation software in the paper "Simulation study on the influence of carburized layer thickness on quenching distortion of 18Cr2Ni4WA steel gear" (Kong Lingli, He Ruijun, Li Guifa. Simulation study on the influence of carburized layer thickness on quenching distortion of 18Cr2Ni4WA steel gear [J]. Metal Heat Treatment, 2021, 46(12): 268-275. DOI: 10.13251 / j.issn.0254-6051.2021.12.045) to analyze the influence of carburized layer on temperature field, stress field and strain field of 18Cr2Ni4WA steel arc gear during heat treatment gas quenching; Cao Zhigang et al. used finite element simulation software in the paper "Finite element simulation study on internal residual stress state of gear in carburized quenching process" (Cao Zhigang et al. Gang, Liu Shijun, Li Yufeng, et al. Finite element simulation study on the residual stress state inside gears during carburizing and quenching process [J]. Internal Combustion Engines and Accessories, 2022, (10): 17-19. DOI: 10.19475 / j.cnki.issn1674-957x.2022.10.010) The finite element method was used to simulate the gear carburizing and quenching process, showing the changes in the temperature field of the quenched specimen and the final formation of the residual stress field; the above papers only considered the influence of the change of a single heat treatment process parameter on the carburizing and quenching heat treatment, because the carburizing and quenching process with multiple variable parameters was numerically simulated to study the influence of heat treatment process parameters on the temperature field, diffusion field, stress / deformation field and microstructure field. Summary of the invention

[0004] The purpose of the present invention is to provide a method for improving the carburizing and quenching heat treatment shape control process by utilizing heat treatment numerical values, and to analyze the influence of heat treatment parameters on the multi-scale whole process of carburizing and quenching heat treatment by utilizing numerical simulation results, so as to perform multi-objective optimization for the carburizing and quenching heat treatment shape control of gears, which are key components of mechanical systems.

[0005] The technical solution to achieve the purpose of the present invention is: a method for improving the carburizing and quenching heat treatment shape control process by using heat treatment values, comprising the following steps:

[0006] Step 1: Establish the finite element analysis model of FZG spur gear carburizing and quenching heat treatment:

[0007] Solidworks modeling software was used to build the FZG-C standard spur gear model and import it into GID. Due to the large amount of calculation required for the simulation of the whole gear and the symmetry of the FZG-C standard spur gear, in order to reduce the amount of calculation, that is, only a single gear tooth needs to be simulated to ensure the accuracy of the simulation, an eight-node hexahedron was used to mesh the gear teeth, and subdivided meshes were performed near the tooth top, tooth bottom, and tooth profile.

[0008] Step 2: Set the boundary conditions and constraints of the FZG spur gear carburizing and quenching heat treatment finite element analysis model: Temperature field boundary conditions: Set the carburizing and quenching heat treatment environment to room temperature, set the thermal conductivity and surface heat transfer coefficient;

[0009] Diffusion field boundary conditions: set the carbon concentration of the carburizing environment to 1.1%, the carbon concentration of the gear tooth surface after carburizing to 0.75%, and set the diffusion coefficient and transfer coefficient;

[0010] Constraints: Typical three-point constraints are performed on the centers of the XZ and YZ planes respectively. The gears can deform freely but no rigid body movement occurs.

[0011] Step 3: Analysis of thermophysical parameters of 20MnCrS5 steel:

[0012] The material simulation software Jmatpro was used to calculate the thermal property curves and CCT curves of 20MnCrS5 steel, including density, Young's modulus, shear modulus, Poisson's ratio, specific heat, etc.

[0013] Thermal simulation experiments were conducted, and the uniaxial loading method was used to study the phase transformation plasticity of 20MnCrS5 steel during martensite-bainite phase transformation. According to the axial expansion curve of the specimen when the martensite-bainite phase transformation occurred under the action of external force, the deformation of the specimen caused by phase transformation plasticity under different temperature applied stress was measured, and the phase transformation plasticity parameters in the Greenwood-Johnson model were determined;

[0014] The heat transfer coefficient determination experiment was carried out, and the cooling capacity of the coolant used in the heat treatment was evaluated using a disc-shaped sample. The disc-shaped sample was heated to a temperature above the austenite phase transition temperature. The material of the test piece was austenite stainless steel SUS303, which would not undergo phase transition during the cooling process, and would not generate latent heat due to phase transition, so the cooling capacity of the coolant could be measured alone;

[0015] Step 4: Establish the carburizing and quenching heat treatment temperature-diffusion-phase transformation-stress-strain field coupling formula:

[0016] The mixed rule equation for carburizing and quenching heat treatment is established. The material point where the structure changes due to phase change inside the material is composed of a mixture of N components such as austenite, pearlite, martensite and carbide. The volume of the first component is recorded as ξ I , then the mechanical and physical properties of the material χ can be expressed as the properties of each component χ I The linear combination equation is:

[0017]

[0018]

[0019] ξA +ξ P +ξ M +ξ C =1

[0020] In the above formula: A , P , M , C are the volume fractions of austenite, pearlite, martensite, and carbide, respectively;

[0021] A heat transfer model for carburizing and quenching heat treatment is established. Based on the first law of thermodynamics and the general expression of heat transfer, the specific heat capacity function and the enthalpy density function are introduced, and the heat transfer equation under the coupling of phase change and stress-strain is obtained:

[0022]

[0023] The boundary conditions for convective heat transfer on the surface are:

[0024] Among them, the second term represents the latent heat generated by elastic work, the third term represents the latent heat generated by inelastic strain, and the fourth term represents the latent heat generated by tissue phase change. σ ij , H, κ and k represent density, specific heat, temperature, elastic strain, plastic strain, flow stress, enthalpy density, hardening coefficient and thermal conductivity respectively, x represents different positions; h T and T W are the unit normal heat transfer coefficient and the temperature of the coolant on the heat transfer boundary respectively;

[0025] A carburizing model for carburizing and quenching heat treatment is established, and the carburizing process is described by the diffusion equation based on Fick's second law. The diffusion equation is:

[0026]

[0027] The external boundary conditions are:

[0028] In the above formula:

[0029] D C =∑D CI (C)ξ I

[0030]

[0031] Among them, C, C W 、r c represent the diffusion mass percentage of carbon, environmental carbon potential and internal source of carbon, β C is the diffusion coefficient of carbon, is the carbon concentration gradient;

[0032] A phase transformation kinetic model of carburizing and quenching heat treatment is established to simulate carburizing and quenching heat treatment. It is assumed that the microstructure consists of three components: austenite, pearlite or bainite and martensite. The formation of pearlite is the diffusion and rearrangement of carbon atoms and iron atoms in austenite to form pearlite. The diffusion-type transformation from austenite to pearlite is calculated based on the Johnson-Mehl relationship. Due to the carburizing and quenching heat treatment process, the carburizing process will perform surface hardening on the gear. It is necessary to consider the effect of carbon content change on diffusion-type transformation. The volume fraction of pearlite ξ p The formula is:

[0033]

[0034]

[0035] f2(C)=exp{-a7(C-C0)}

[0036] f3(σ)=exp(a8σ m )

[0037] Among them: a i (=0, 1, 2, 3, 4, 5, 6, 8) are the transformation kinetic parameters, a7 is the carbon-dependent change parameter: C and C0 are the current and initial carbon contents, respectively;

[0038] The non-diffusion transformation from austenite to martensite is based on Magee kinetic calculations, and the formula for the volume change of martensite phase transformation is: M =1-exp{φ1T+φ2(C-C0)+φ3σ m +φ4σ e +φ5};

[0039] Among them, σ m and σ e are the average stress and equivalent stress; the φ1~φ5 coefficients are determined by experiments and are material parameters;

[0040] The stress-strain constitutive model of carburizing and quenching heat treatment is established. According to the constitutive equation of the material, the total strain rate of the material under phase change is expressed as elastic strain Rate, plastic strain Rate, thermal strain Rate, phase change strain Rate, phase transformation plastic strain rate and diffusion-induced strain rate The sum of the rates is expressed as:

[0041]

[0042]

[0043] Among them, E, ν, α, and β represent the global forms of Young's modulus, Poisson's ratio, thermal expansion coefficient, and phase change expansion coefficient of the material, respectively. ij With δ ij denote stress and strain respectively, and F represent the hardening function and yield function respectively, a is the lattice parameter of austenite, and Represent the carbon content at time t0 and t1 respectively;

[0044] A carburizing and quenching heat treatment hardening criterion model is established. The hardness after quenching depends on its material, metallographic structure and cooling rate. The hardness formula is: HV = ξ M ·HV M +ξ B ·HV B +ξ F ·HV F +ξ P ·HV P ;

[0045] The formula for calculating martensite hardness is: HV M =N·M T ;

[0046] The formula for calculating bainite hardness is: HV B =N·B T ;

[0047] The hardness of ferrite and pearlite is: HV P =HV F =N·F T ;

[0048] Since the residual austenite (RA) on the surface of the part after carburizing and quenching will reduce the hardness of the high-carbon martensite, the hardness formula of the retained austenite is:

[0049] Correction to the formula for calculating martensite hardness: HRC correctedM =HRC M -ΔHRC;

[0050] Correction to hardness formula: HV correct =V M ·HV correctedM +V B ·HV B +V F ·HV F +V P ·HV P ;

[0051] Step 5: Call the thermophysical properties of 20MnCrS5 steel, CCT curve and carburizing and quenching heat treatment temperature-diffusion-phase transformation-stress-strain field coupling formula, and combine it with the FZG spur gear finite element analysis model to write a full-process simulation program for carburizing and quenching heat treatment. Use COSMAP heat treatment simulation software to perform numerical simulation on four different carburizing and quenching heat treatment processes. Take the surface residual stress and deformation as the target, analyze the simulation results of carburizing and quenching heat treatment, and determine the optimal carburizing and quenching heat treatment process.

[0052] According to the optimal carburizing and quenching heat treatment process, the quenching method was changed (with quenching angles of 0° and 60°), and the COSMAP heat treatment simulation software was reused to simulate the modified process. The process was experimented, and the simulation results were compared and analyzed with the experimental results.

[0053] According to the simulation results (retained austenite, martensite, pearlite, temperature, carbon concentration, stress, hardness and deformation distribution diagram) and experimental measurement results (hardness, residual stress, retained austenite), based on the continuum thermodynamics theory and phase transformation kinetics theory, the influence of carburizing and quenching heat treatment process parameters on the temperature-diffusion-phase transformation-stress-strain multi-coupled field and mechanical properties is analyzed, and multi-objective optimization is carried out for the carburizing and quenching heat treatment morphological control of gears, which are key components of mechanical systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 The present invention is a flow chart of a method for improving the carburizing and quenching heat treatment shape control process by utilizing heat treatment values.

[0055] Figure 2a This is the finite element model diagram of FZG spur gear;

[0056] Figure 2b This is the single tooth finite element model diagram of FZG spur gear;

[0057] Figure 2c This is the loading diagram of boundary conditions for FZG spur gear;

[0058] Figure 2d It is the constraint condition loading diagram of FZG spur gear;

[0059] Figure 2e This is the mesh division diagram of FZG spur gear;

[0060] Figure 3a It is the density parameter curve of 20MnCrS5 steel;

[0061] Figure 3b It is the Young's modulus parameter curve of 20MnCrS5;

[0062] Figure 3c It is the parameter curve of Poisson's ratio of 20MnCrS5;

[0063] Figure 3d It is the shear modulus parameter curve of 20MnCrS5;

[0064] Figure 3e It is the specific heat parameter curve of 20MnCrS5;

[0065] Figure 3f It is the latent heat parameter curve of 20MnCrS5;

[0066] Figure 4 It is the CCT curve of 20MnCrS5 steel;

[0067] Figure 5a It is the convection heat transfer coefficient diagram of quenching oil S changing with temperature;

[0068] Figure 5b It is the convection heat transfer coefficient diagram of quenching oil H changing with temperature;

[0069] Figure 6 Schematic diagram of quenching angle.

[0070] Figure 7 This is the simulation result diagram of the second oil H-horizontal quenching heat treatment process.

[0071] Figure 8 This is the simulation result diagram of the second oil H-oblique quenching heat treatment process.

[0072] Fig. 9 Schematic diagram of residual stress test points after carburizing and quenching.

[0073] Fig.10 Schematic diagram of hardness test points after carburizing and quenching. DETAILED DESCRIPTION

[0074] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the invention of this article is further described in detail below in conjunction with embodiments and drawings. The illustrative embodiments of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0075] Combination Figure 1 The method of improving the carburizing and quenching heat treatment shape control process by using heat treatment values ​​described in the present invention comprises the following specific steps:

[0076] Step 1: Establish the finite element analysis model of FZG spur gear carburizing and quenching heat treatment:

[0077] Solidworks modeling software was used to build the FZG-C standard spur gear model and import it into GID. Due to the large amount of calculation required for the simulation of the whole gear and the symmetry of the FZG-C standard spur gear, in order to reduce the amount of calculation, that is, only a single gear tooth needs to be simulated to ensure the accuracy of the simulation, an eight-node hexahedron was used to mesh the gear teeth, and subdivided meshes were performed near the tooth top, tooth bottom, and tooth profile.

[0078] Step 2: Set the boundary conditions and constraints of the FZG spur gear carburizing and quenching heat treatment finite element analysis model:

[0079] Temperature field boundary conditions: Set the carburizing and quenching heat treatment environment to room temperature, and set the thermal conductivity and surface heat transfer coefficient;

[0080] Diffusion field boundary conditions: set the carbon concentration of the carburizing environment to 1.1%, the carbon concentration of the gear tooth surface after carburizing to 0.75%, and set the diffusion coefficient and transfer coefficient;

[0081] Constraints: Typical three-point constraints are performed on the centers of the XZ and YZ planes, respectively. The gear can deform freely but does not move rigidly.

[0082] Step 3: Analysis of thermophysical parameters of 20MnCrS5 steel:

[0083] The material simulation software Jmatpro was used to calculate the thermal property curves and CCT curves of 20MnCrS5 steel, including density, Young's modulus, shear modulus, Poisson's ratio, specific heat, etc.

[0084] Thermal simulation experiments were conducted, and the uniaxial loading method was used to study the phase transformation plasticity of 20MnCrS5 steel during martensite-bainite phase transformation. According to the axial expansion curve of the specimen when the martensite-bainite phase transformation occurred under the action of external force, the deformation of the specimen caused by phase transformation plasticity under different temperature applied stress was measured, and the phase transformation plasticity parameters in the Greenwood-Johnson model were determined;

[0085] The heat transfer coefficient determination experiment was carried out, and the cooling capacity of the coolant used in the heat treatment was evaluated using a disc-shaped sample. The disc-shaped sample was heated to a temperature above the austenite phase transition temperature. The material of the test piece was austenite stainless steel SUS303, which would not undergo phase transition during the cooling process, and would not generate latent heat due to phase transition, so the cooling capacity of the coolant could be measured alone;

[0086] Step 4: Establish the carburizing and quenching heat treatment temperature-diffusion-phase transformation-stress-strain field coupling formula:

[0087] The mixed rule equation for carburizing and quenching heat treatment is established. The material point where the structure changes due to phase change inside the material is composed of a mixture of N components such as austenite, pearlite, martensite and carbide. The volume of the first component is recorded as ξ I , then the mechanical and physical properties of the material χ can be expressed as the properties of each component χ I The linear combination equation is:

[0088]

[0089] ξ A +ξ P +ξ M +ξ C =1

[0090] In the above formula: A , P , M , C are the volume fractions of austenite, pearlite, martensite, and carbide, respectively;

[0091] A heat transfer model for carburizing and quenching heat treatment is established. Based on the first law of thermodynamics and the general expression of heat transfer, the specific heat capacity function and the enthalpy density function are introduced, and the heat transfer equation under the coupling of phase change and stress-strain is obtained:

[0092]

[0093] The boundary conditions for convective heat transfer on the surface are:

[0094] Among them, the second term represents the latent heat generated by elastic work, the third term represents the latent heat generated by inelastic strain, and the fourth term represents the latent heat generated by tissue phase change. σ ij , H, κ and k represent density, specific heat, temperature, elastic strain, plastic strain, flow stress, enthalpy density, hardening coefficient and thermal conductivity respectively, x represents different positions; h T and T W are the unit normal heat transfer coefficient and the temperature of the coolant on the heat transfer boundary respectively;

[0095] A carburizing model for carburizing and quenching heat treatment is established, and the carburizing process is described by the diffusion equation based on Fick's second law. The diffusion equation is:

[0096]

[0097] The external boundary conditions are:

[0098] In the above formula:

[0099] D C =∑DCI (C)ξ I

[0100]

[0101] Among them, C, C W 、r c represent the diffusion mass percentage of carbon, environmental carbon potential and internal source of carbon, β C is the diffusion coefficient of carbon, is the carbon concentration gradient;

[0102] A phase transformation kinetic model of carburizing and quenching heat treatment is established to simulate carburizing and quenching heat treatment. It is assumed that the microstructure consists of three components: austenite, pearlite or bainite and martensite. The formation of pearlite is the diffusion and rearrangement of carbon atoms and iron atoms in austenite to form pearlite. The diffusion-type transformation from austenite to pearlite is calculated based on the Johnson-Mehl relationship. Due to the carburizing and quenching heat treatment process, the carburizing process will perform surface hardening on the gear. It is necessary to consider the effect of carbon content change on diffusion-type transformation. The volume fraction of pearlite ξ is corrected. p The formula is:

[0103]

[0104] Then the Johnson-Mehl kinetic formula needs to be modified:

[0105]

[0106]

[0107] f2(C)=exp{-a7(C-C0)}

[0108] f3(σ)=exp(a8σ m )

[0109] Among them: a i (=0, 1, 2, 3, 4, 5, 6, 8) are the transformation kinetic parameters, a7 is the carbon-dependent change parameter: C and C0 are the current and initial carbon contents, respectively;

[0110] The non-diffusion type transformation from austenite to martensite is based on Magee kinetic calculation. Since the carburizing process will harden the surface of the gear, it is necessary to consider the effect of carbon content change on the diffusion type transformation. Therefore, the kinetic formula needs to be modified. The formula for the volume change of martensite phase transformation is: ξ M =1-exp{φ1T+φ2(C-C0)+φ3σ m +φ4σ e +φ5};

[0111] Among them, σ m and σ e are the average stress and equivalent stress; the φ1~φ5 coefficients are determined by experiments and are material parameters;

[0112] The stress-strain constitutive model of carburizing and quenching heat treatment is established. According to the constitutive equation of the material, the total strain rate of the material under phase change is expressed as elastic strain Rate, plastic strain Rate, thermal strain Rate, phase change strain Rate, phase transformation plastic strain rate and diffusion-induced strain rate The sum of the rates is expressed as:

[0113]

[0114] Among them, E, ν, α, and β represent the global forms of Young's modulus, Poisson's ratio, thermal expansion coefficient, and phase change expansion coefficient of the material, respectively. ij With δ ij denote stress and strain respectively, and F represent the hardening function and yield function respectively, a is the lattice parameter of austenite, and Represent the carbon content at time t0 and t1 respectively;

[0115] A carburizing and quenching heat treatment hardening criterion model is established. The hardness after quenching depends on its material, metallographic structure and cooling rate. The hardness formula is: HV = ξ M ·HV M +ξ B ·HV B +ξ F ·HV F +ξ P ·HV P ;

[0116] The formula for calculating martensite hardness is: HV M =N·M T ;

[0117] The formula for calculating bainite hardness is: HV B =N·B T ;

[0118] The hardness of ferrite and pearlite is: HV P =HV F =N·F T ;

[0119] Since the residual austenite (RA) on the surface of the part after carburizing and quenching will reduce the hardness of the high-carbon martensite, the hardness formula of the retained austenite is:

[0120] Correction to the formula for calculating martensite hardness: HRC correctedM =HRC M -ΔHRC;

[0121] Correction to hardness formula: HV correct =V M ·HV correctedM +V B ·HV B +V F ·HV F +V P ·HV P ;

[0122] Step 5: Call the thermophysical properties of 20MnCrS5 steel, CCT curve and carburizing and quenching heat treatment temperature-diffusion-phase transformation-stress-strain field coupling formula, and combine it with the FZG spur gear finite element analysis model to write a full-process simulation program for carburizing and quenching heat treatment. Use COSMAP heat treatment simulation software to perform numerical simulation on four different carburizing and quenching heat treatment processes. Take the surface residual stress and deformation as the target, analyze the simulation results of carburizing and quenching heat treatment, and determine the optimal carburizing and quenching heat treatment process.

[0123] According to the optimal carburizing and quenching heat treatment process, the quenching method was changed (with quenching angles of 0° and 60°), and the COSMAP heat treatment simulation software was reused to simulate the modified process. The process was experimented, and the simulation results were compared and analyzed with the experimental results.

[0124] According to the simulation results (retained austenite, martensite, pearlite, temperature, carbon concentration, stress, hardness and deformation distribution diagram) and experimental measurement results (hardness, residual stress, retained austenite), based on the continuum thermodynamics theory and phase transformation kinetics theory, the influence of carburizing and quenching heat treatment process parameters on the temperature-diffusion-phase transformation-stress-strain multi-coupled field and mechanical properties is analyzed, and multi-objective optimization is carried out for the carburizing and quenching heat treatment morphological control of gears, which are key components of mechanical systems.

[0125] Example 1

[0126] The following is an analysis of the numerical simulation calculation of the whole process of multi-process carburizing and quenching heat treatment of low-carbon alloy steel 20MnCrS5 gear steel FZG spur gear. The chemical composition (wt.%) of low-carbon alloy steel 20MnCrS5 is as follows:

[0127] Table 1

[0128] Fe C Si Mn Cr Ni Mo V N P S Al Ti 96.854 0.2 0.08 1.32 1.22 0.04 0.008 0.007 0.2 0.014 0.024 0.032 0.0013

[0129] Step 1: Establish the finite element calculation model of FZG spur gear carburizing and quenching heat treatment

[0130] 1.1: According to the actual working conditions, use Solidworks modeling software to establish the FZG-C standard spur gear model and import the GID, such as Figure 2a Since the simulation of the whole gear requires a lot of calculations and the FZG-C standard spur gear is symmetrical, in order to reduce the amount of calculations, only a single gear tooth needs to be simulated to ensure the accuracy of the simulation. Figure 2b Use eight-node hexahedron to mesh the gear teeth, such as Figure 2e , refine the mesh around the tooth top, tooth bottom and tooth profile, as shown in the figure. The number of nodes of the model is 4410 and the number of units is 3690. Export the finite element model in .nas file format and import this file into COSMAP heat treatment simulation software; 1.2: Set the boundary conditions and constraints of the finite element analysis model of FZG spur gear carburizing and quenching heat treatment, such as Figure 2c and Figure 2d , Temperature field boundary conditions, set the carburizing and quenching heat treatment environment to room temperature, set the thermal conductivity and surface heat transfer coefficient, diffusion field boundary conditions: set the carbon concentration of the carburizing environment to 1.1%, the carbon concentration of the gear tooth surface after carburizing to 0.75%, set the diffusion coefficient and transfer coefficient;

[0131] Step 2: Analysis of thermophysical properties of low carbon alloy steel 20MnCrS5

[0132] 1.1: Use the material simulation software Jmatpro to calculate the density, Young's modulus, shear modulus, Poisson's ratio, specific heat and other thermal properties and CCT curves of 20MnCrS5 steel, such as Figure 3a to Figure 3f ,as well as Figure 4 , write the low carbon alloy steel 20MnCrS5 material .dat file and import it into cosmap software;

[0133] 1.2: Conduct thermal simulation experiments and use uniaxial loading methods to study the phase transformation plasticity of 20MnCrS5 steel during martensite and bainite phase transformation. According to the axial expansion curve of the specimen when the martensite and bainite phase transformation occurs under external force, the deformation of the specimen caused by phase transformation plasticity under different temperature applied stress is measured, and the phase transformation plasticity parameters in the Greenwood-Johnson model are determined; as shown in the following table:

[0134] Table 2

[0135] 20MnCrS5 Austenite transformation Bainite transformation Martensitic transformation Elastic modulus (G·Pa) 198.718 211.266 211.266 Poisson's ratio 0.2938 0.28952 0.28952 Density (g / cm^3) 8.09799 7.83827 7.82465 Yield limit 201.62 798.23 947.64 Ultimate Strength 220.62 1264.79 1508.64 Linear expansion coefficient 3.70E-05 3.43E-05 2.12E-05 Phase change expansion coefficient -2.43E-03 2.44E-03 3.79E-03 Phase transformation plasticity coefficient 9.96E-05 6.67E-05 8.1E-05

[0136] 1.3: Conduct a heat transfer coefficient determination experiment. Use a disc-shaped sample to evaluate the cooling capacity of the coolant used in the heat treatment. Heat the disc-shaped sample to a temperature above the austenite phase transition. The material of the test piece is austenitic stainless steel SUS303, which will not undergo phase transition during the cooling process. There will be no latent heat due to phase transition, so the cooling capacity of the coolant can be measured separately. Use quenching oil S and quenching oil H to cool the four heat treatment processes respectively. The quenching oil convection heat transfer coefficient is as follows: Figure 5a and Figure 5b ;

[0137] Step 3: Establish a simulation model for the entire process of carburizing and quenching heat treatment

[0138] 1.1: Establish the coupled formula of temperature-diffusion-phase transformation-stress-strain field for carburizing and quenching heat treatment

[0139] The mixed rule equation for carburizing and quenching heat treatment is established. The material point where the structure changes due to phase change inside the material is composed of a mixture of N components such as austenite, pearlite, martensite and carbide. The volume of the first component is recorded as ξ I , then the mechanical and physical properties of the material χ can be expressed as the properties of each component χ I The linear combination equation is:

[0140]

[0141] ξ A +ξ P +ξ M +ξ C =1

[0142] In the above formula: A , P , M , C are the volume fractions of austenite, pearlite, martensite and carbide, respectively.

[0143] A heat transfer model for carburizing and quenching heat treatment is established. Based on the first law of thermodynamics and the general expression of heat transfer, the specific heat capacity function and the enthalpy density function are introduced, and the heat transfer equation under the coupling of phase change and stress-strain is obtained:

[0144]

[0145] The boundary conditions for convective heat transfer on the surface are:

[0146] Among them, the second term represents the latent heat generated by elastic work, the third term represents the latent heat generated by inelastic strain, and the fourth term represents the latent heat generated by tissue phase change. σ ij, H, κ and k represent density, specific heat, temperature, elastic strain, plastic strain, flow stress, enthalpy function, hardening coefficient and thermal conductivity respectively, x represents different positions; h T and T W are the unit normal heat transfer coefficient and the temperature of the coolant on the heat transfer boundary respectively;

[0147] A carburizing model for carburizing and quenching heat treatment is established, and the carburizing process is described by the diffusion equation based on Fick's second law. The diffusion equation is:

[0148]

[0149] The external boundary conditions are: In the above formula:

[0150] D C =∑D CI (C)ξ I

[0151]

[0152] Among them, C, C W 、r c represent the diffusion mass percentage of carbon, environmental carbon potential and internal source of carbon, β C is the diffusion coefficient of carbon, is the carbon concentration gradient;

[0153] A phase transformation kinetic model of carburizing and quenching heat treatment is established to simulate carburizing and quenching heat treatment. It is assumed that the microstructure consists of three components: austenite, pearlite or bainite and martensite. The formation of pearlite is the diffusion and rearrangement of carbon atoms and iron atoms in austenite. The diffusion-type transformation from austenite to pearlite is calculated based on the modified Johnson-Mehl relationship. The volume fraction of pearlite ξ p The formula is:

[0154] Since the carburizing and quenching heat treatment process is used, the carburizing process will harden the surface of the gear. It is necessary to consider the factor of carbon content change on the diffusion type transformation, so the Johnson-Mehl kinetic formula needs to be corrected:

[0155]

[0156] f2(C)=exp{-a7(C-C0)}

[0157] f3(σ)=exp(a8σ m )

[0158] Among them: a i(=0, 1, 2, 3, 4, 5, 6, 8) are the transformation kinetic parameters, a7 is the carbon-dependent change parameter: C and C0 are the current and initial carbon contents, respectively;

[0159] The non-diffusion type transformation from austenite to martensite is based on Magee kinetic calculation. Since the carburizing process will harden the surface of the gear, it is necessary to consider the effect of carbon content change on the diffusion type transformation. Therefore, the kinetic formula needs to be modified. The formula for the volume change of martensite phase transformation is: ξ M =1-exp{φ1T+φ2(C-C0)+φ3σ m +φ4σ e +φ5}

[0160] Among them, σ m and σ e are the average stress and equivalent stress; the φ1~φ5 coefficients are determined by experiments and are material parameters;

[0161] The stress-strain constitutive model of carburizing and quenching heat treatment is established. According to the constitutive equation of the material, the total strain rate of the material under phase change is expressed as elastic strain Rate, plastic strain Rate, thermal strain Rate, phase change strain Rate, phase transformation plastic strain rate and diffusion-induced strain rate The sum of the rates is expressed as:

[0162]

[0163]

[0164] Among them, E, ν, α, and β represent the global forms of Young's modulus, Poisson's ratio, thermal expansion coefficient, and phase change expansion coefficient of the material, respectively. ij With δ ij denote stress and strain respectively, and F represent the hardening function and yield function respectively, a is the lattice parameter of austenite, and Represent the carbon content at time t0 and t1 respectively;

[0165] A carburizing and quenching heat treatment hardening criterion model is established. The hardness after quenching depends on its material, metallographic structure and cooling rate. The hardness formula is: HV = ξ M ·HV M +ξ B ·HV B +ξ F ·HV F +ξ P ·HV P ;

[0166] The formula for calculating martensite hardness is: HV M =N·M T ;

[0167] The formula for calculating bainite hardness is: HV B =N·B T ;

[0168] The hardness of ferrite and pearlite is: HV P =HV F =N·F T ;

[0169] Since the residual austenite (RA) on the surface of the part after carburizing and quenching will reduce the hardness of the high-carbon martensite, the hardness formula of the retained austenite is:

[0170] Correction to the formula for calculating martensite hardness: HRC correctedM =HRC M -ΔHRC;

[0171] Correction to hardness formula: HV correct =V M ·HV correctedM +V B ·HV B +V F ·HV F +V P ·HV P ;

[0172] To establish the coupled model of temperature-diffusion-phase transformation-stress-strain field of over-carburizing and quenching heat treatment, based on the material thermophysical property parameters, write the .pos file of the whole process simulation of carburizing and quenching heat treatment and import it into COSMAP software;

[0173] Step 4: Simulation analysis of the entire carburizing and quenching process:

[0174] The model .nas file, material .dat file and heat treatment process .pos file are run through COSMAP (in order to reduce the calculation time here, the model uses a cylindrical model for simulation), and four carburizing and quenching heat treatment processes are simulated under two quenching oils to obtain the distribution of stress and strain fields. With the optimal deformation and residual stress as the goal, the optimal heat treatment process is selected. The simulation of stress and strain fields of four heat treatment processes is shown in the following table:

[0175] Table 3

[0176]

[0177] According to the stress-strain distribution field of the above four carburizing and quenching heat treatment processes, it can be seen that in terms of residual stress, the residual compressive stress on the surface of the sample quenched with oil H is the largest; in terms of deformation, the deformation of the sample quenched with oil H is the smallest, which shows the importance of controlling the cooling rate. If the cooling rate is too fast, it will cause the deformation of the parts to increase, and the surface residual stress will be residual tensile stress; Process 2 increases the pre-quenching temperature by 10 degrees compared with Process 1, and at the same time increases the carburizing time. The simulation results show that the deformation and residual stress of the parts heat treated with Process 2 are better than those of Process 1; Process 3 increases the pre-treatment process and the austenitization time, which makes the grains coarse and leads to poor results; Among the four processes using H oil, Process 2 and Process 4 have the largest residual compressive stress and the smallest deformation; Compared with Process 2 and Process 4, the surface residual compressive stress of Process 2 is larger than that of Process 4, but the deformation is smaller than that of Process 4. In summary, Process 2 using Oil H is the optimal heat treatment process;

[0178] Step 5: Simulation analysis and experimental verification of the whole process of carburizing and quenching by quenching angle change:

[0179] 1.1: Low carbon alloy steel 20MnCrS5 completes carburization after entering the strong permeation and diffusion stages, and then is kept warm for a period of time for horizontal and oblique quenching, such as Figure 6 The heat treatment simulation software COSMAP was used to simulate the horizontal and inclined quenching of the heat treatment process 2 using oil H. The horizontal quenching simulation results are shown in Figure 7 , the simulation results of inclined quenching are as follows Figure 8 The comparative analysis results are as follows:

[0180] Table 4

[0181] Horizontal quenching Inclined (60°) quenching Maximum deformation(mm) 0.022966 0.015799 Minimum deformation (mm) 9.32e-05 8.95e-0.5 Maximum deformation of gear pitch circle (mm) 0.016603 0.011603 Minimum deformation of gear pitch circle (mm) 0.008658 0.003188 Maximum tensile residual stress (MPa) 479.89 466.38 Minimum compressive residual stress (MPa) -300.88 -310.36 Maximum hardness (HV) 701.55 738.34 Maximum content of martensite (%) 99.032 99.032 Maximum bainite content (%) 34.643 40.299 Maximum austenite content (%) 7.661 7.49

[0182] 1.2: Carry out X-ray residual stress test on each point of FZG spur gear after carburizing and quenching heat treatment. Fig. 9 , the residual stress test results are as follows:

[0183] Table 5

[0184]

[0185] 1.3: Vickers hardness test of each point of FZG spur gear after carburizing and quenching heat treatment. Fig.10 ; The hardness test results are as follows:

[0186] Table 6

[0187]

[0188] 1.4: The surface and core structure distribution of FZG spur gears after carburizing and quenching heat treatment were measured. The results of the structure distribution test are as follows:

[0189] Table 7

[0190]

[0191] 1.5: According to the simulation and measurement of residual stress, Vickers hardness, and organization distribution, the whole process of carburizing and quenching heat treatment of low-carbon alloy steel 20MnCrS5 is simulated and predicted by COSMAP heat treatment software, and the organization distribution, hardness and residual stress of the two are well matched by comparing the experimental measurement with the simulation results, which proves the correctness of the finite element model and also explains the strengthening mechanism of heat treatment from a theoretical perspective. After carburizing and quenching heat treatment, residual compressive stress is shown between teeth and at the center of the pitch circle. Compared with the vertical position, the residual compressive stress on the gear surface after oblique carburizing and quenching is greater. After the FZG gear is carburized and quenched, the hardness of each part of the gear is improved to varying degrees, among which the surface hardness is as high as about 750HV. Compared with the surface hardness at the pitch circle of the gear, the hardness at the pitch circle of the gear after oblique quenching is higher than that of the horizontal quenching. In summary, the best carburizing and quenching heat treatment process is heat treatment process 2-oblique quenching using quenching oil H.

[0192] The present invention provides a method for improving the carburizing and quenching heat treatment shape control process by utilizing heat treatment numerical values, and uses numerical simulation results to analyze the influence of heat treatment parameters on the multi-scale full process of carburizing and quenching heat treatment, so as to perform multi-objective optimization for the carburizing and quenching heat treatment shape control of gears, which are key components of mechanical systems.

Claims

1. A method for improving the shape control process of carburizing and quenching heat treatment by using heat treatment numerical values, characterized in that: The specific steps are as follows: Step 1: Establish the finite element analysis model of FZG spur gear carburizing and quenching heat treatment; Solidworks modeling software was used to build the FZG-C standard spur gear model, and GID was imported. Eight-node hexahedron was used to mesh the gear teeth, and subdivided meshes were made near the tooth top, tooth bottom, and tooth profile. Step 2: Set the boundary conditions and constraints of the finite element analysis model for carburizing and quenching heat treatment of FZG spur gears; Step 3: Analysis of thermophysical parameters of 20MnCrS5 steel; Step 4: Establish the coupled formula of temperature-diffusion-phase transformation-stress-strain field for carburizing and quenching heat treatment; The specific method is: The mixing rule equation for carburizing and quenching heat treatment is established. According to the material point where the structure changes due to phase change inside the material, it is composed of a mixture of N components. The volume of the first component is recorded as ξ I , then the mechanical and physical properties of the material χ are expressed as the properties of each component χ I The linear combination equation is: In the above formula: A , P , M , C are the volume fractions of austenite, pearlite, martensite, and carbide, respectively; A heat transfer model for carburizing and quenching heat treatment is established. Based on the first law of thermodynamics and the general expression of heat transfer, the specific heat capacity function and the enthalpy density function are introduced to obtain the heat transfer equation under the coupling of phase change and stress-strain: The boundary conditions for convective heat transfer on the surface are: Among them, the second term represents the latent heat generated by elastic work, the third term represents the latent heat generated by inelastic strain, and the fourth term represents the latent heat generated by tissue phase change. σ ij , H, κ and k represent density, specific heat, temperature, elastic strain, plastic strain, flow stress, enthalpy density, hardening coefficient and thermal conductivity respectively, x represents different positions, h T and T W are the unit normal heat transfer coefficient and the temperature of the coolant on the heat transfer boundary respectively; A carburizing model for carburizing and quenching heat treatment is established, and the carburizing process is described by the diffusion equation based on Fick's second law. The diffusion equation is: The external boundary conditions are: In the above formula: Using the mixing rule to obtain the global diffusion rate D C expression: D C =∑D CI (C)ξ I Among them, C, C W 、r c represent the diffusion mass percentage of carbon, environmental carbon potential and internal source of carbon, β C is the diffusion coefficient of carbon, is the carbon concentration gradient; A phase transformation kinetic model of carburizing and quenching heat treatment is established to simulate carburizing and quenching heat treatment. It is assumed that the microstructure consists of three components: austenite, pearlite or bainite and martensite. The formation of pearlite is the diffusion and rearrangement of carbon atoms and iron atoms in austenite to form pearlite. The diffusion-type transformation from austenite to pearlite is calculated based on the Johnson-Mehl relationship. Due to the carburizing and quenching heat treatment process, the carburizing process will perform surface hardening on the gear. It is necessary to consider the effect of carbon content change on diffusion-type transformation. The volume fraction of pearlite ξ p The formula is: In the above formula: f2(C)=exp{-a7(C-C0)}、 f3(σ)=exp(a8σ m ) Among them: a0, a1, a2, a3, a4, a5, a6, a8 are transformation kinetic parameters, a7 is a parameter that depends on the carbon change: C and C0 are the current and initial carbon contents, respectively; The non-diffusion transformation from austenite to martensite is based on Magee kinetic calculations, and the formula for the volume change of martensite phase transformation is: x M =1-exp{φ1T+φ2(C-C0)+φ3σ m +φ4σ e +φ5} Among them, σ m and σ e are the average stress and equivalent stress; the φ1~φ5 coefficients are determined by experiments and are material parameters; The stress-strain constitutive model of carburizing and quenching heat treatment is established. According to the constitutive equation of the material, the total strain rate of the material under phase change is expressed as elastic strain Rate, plastic strain Rate, thermal strain Rate, phase change strain Rate, phase transformation plastic strain rate and diffusion-induced strain rate The sum of the rates is expressed as: Among them, E, ν, α, and β represent the global forms of Young's modulus, Poisson's ratio, thermal expansion coefficient, and phase change expansion coefficient of the material, respectively. ij With δ ij denote stress and strain respectively, and F represent the hardening function and yield function respectively, a is the lattice parameter of austenite, and Represent the carbon content at time t0 and t1 respectively; A carburizing and quenching heat treatment hardening criterion model is established. The hardness after quenching depends on its material, metallographic structure and cooling rate. The hardness formula is: HV=ξ M ·HV M +ξ B ·HV B +ξ F ·HV F +ξ P ·HV P The formula for calculating martensite hardness is: HV M =N·M T The calculation formula of bainite hardness is: HV B =N·B T The hardness of ferrite and pearlite is: What? P =HV F =N·F T The hardness formula of retained austenite is: Correction to the calculation formula of martensite hardness: HRC correctedM =HRC M -△HRC Correction to the hardness formula: HV correct =V M ·HV correctedM +V B ·HV B +V F ·HV F +V P ·HV P Step 5: Call the thermophysical property parameters, CCT curve and carburizing and quenching heat treatment temperature-diffusion-phase transformation-stress-strain field coupling formula of 20MnCrS5 steel, and combine with the FZG spur gear finite element analysis model to write a simulation program for the whole process of carburizing and quenching heat treatment. Use COSMAP heat treatment simulation software to perform numerical simulation on four different carburizing and quenching heat treatment processes. Take the surface residual stress and deformation as the target, analyze the simulation results of carburizing and quenching heat treatment, and determine the optimal carburizing and quenching heat treatment process. According to the optimal carburizing and quenching heat treatment process, the quenching method was changed, and the modified process was simulated using the COSMAP heat treatment simulation software at quenching angles of 0° and 60°. The process was also tested, and the simulation results were compared and analyzed with the experimental results. According to the simulation results and experimental measurement results, based on the continuum thermodynamics theory and phase transformation kinetics theory, the influence of carburizing and quenching heat treatment process parameters on the temperature-diffusion-phase transformation-stress-strain multi-coupled field and mechanical properties is analyzed, and multi-objective optimization is carried out for the carburizing and quenching heat treatment morphological control of gears, which are key components of mechanical systems.

2. The method for improving the carburizing and quenching heat treatment shape control process by using heat treatment values ​​according to claim 1, characterized in that: The boundary conditions and constraints in the second step are: Temperature field boundary conditions: Set the carburizing and quenching heat treatment environment to room temperature, and set the thermal conductivity and surface heat transfer coefficient; Diffusion field boundary conditions: set the carbon concentration of the carburizing environment to 1.1%, the carbon concentration of the gear tooth surface after carburizing to 0.75%, and set the diffusion coefficient and transfer coefficient; Constraints: Typical three-point constraints are performed on the centers of the XZ and YZ planes respectively. The gears can deform freely but no rigid body movement occurs.

3. The method for improving the carburizing and quenching heat treatment shape control process by using heat treatment values ​​according to claim 1, characterized in that: The specific method of the third step is: The material simulation software Jmatpro was used to calculate the density, Young's modulus, shear modulus, Poisson's ratio, specific heat thermal property curve and CCT curve of 20MnCrS5 steel; Thermal simulation experiments were conducted, and the uniaxial loading method was used to analyze the phase transformation plasticity of 20MnCrS5 steel during martensite-bainite phase transformation. According to the axial expansion curve of the specimen when the martensite-bainite phase transformation occurred under the action of external force, the deformation of the specimen caused by phase transformation plasticity under different temperatures was measured to determine the phase transformation plasticity parameters in the Greenwood-Johnson model. The heat transfer coefficient determination experiment was carried out, and the cooling capacity of the coolant used in the heat treatment was evaluated using a disc-shaped sample. The disc-shaped sample was heated to a temperature above the austenite phase transformation temperature. The material of the test piece was austenitic stainless steel SUS303, which would not undergo phase transformation during the cooling process and would not generate latent heat due to phase transformation, so the cooling capacity of the coolant could be measured alone.

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