A simulation experimental method for heat treatment strengthening of steel gear surface

Abstract

The present invention provides a simulation experimental method for surface heat treatment strengthening of steel gears. The commonly used automotive gear steel 20MnCr5 is taken as the research object, and the gear production and processing are carried out through the actual heat treatment process of automotive gear manufacturing. Considering the multi-field coupling relationship, a carburizing and quenching multi-field coupling model is established. The dynamic process of carburizing and quenching of FZG spur gears is numerically simulated, and the change law and distribution difference of each physical field in the carburizing and quenching process are analyzed. The difference in the post-heat performance of two 20MnCr5 steel gears with different chemical element contents is compared. The influence of various process factors on gear performance during carburizing and quenching is analyzed, and a carburizing and quenching process optimization plan is formulated with hardness, deformation, and carburized layer depth as optimization targets. The optimization plan and evaluation method of the gear carburizing and quenching process are formed. The gear optimization process is efficiently obtained, the number of unnecessary gear trial production times is reduced, the production cost of the enterprise is reduced in production practice, and the economic benefits are improved.

Description

Technical Field

[0001] The invention relates to an engineering application simulation and test verification method, and in particular to a simulation experimental method for surface heat treatment strengthening of a steel gear. Background Art

[0002] At present, carburizing and quenching are widely used for surface strengthening treatment of automotive gears. However, in the production and application of gears, excessive heat treatment deformation, insufficient hardness, and short fatigue life have become common problems. The above problems can be improved by adjusting the heat treatment process. However, the formulation of heat treatment process often needs to consider factors such as steel type, size, and use requirements of gears and other parts. The process is relatively complicated. The reasonable regulation of key gear properties such as hardness, carburized layer depth, and deformation during carburizing and quenching has become one of the technical difficulties. The traditional method often uses the "trial and error method" to obtain the optimized process, which requires a lot of trial production work and cannot be quantitatively and accurately controlled. It is not only time-consuming and labor-intensive, but also greatly increases the manufacturing cost, which does not meet the current high-precision and high-efficiency intelligent manufacturing requirements. With the rapid development of the international automotive parts manufacturing industry towards digitalization and intelligence, the manufacturing industry is changing from a traditional hardware-based industry to an industry centered on numerical simulation technology and solutions. In recent years, computer technology and simulation software for heat treatment processes have been gradually introduced and developed. The numerical simulation method can provide dynamic results output for the temperature field, phase change field, and stress-strain field during carburizing and quenching, and obtain results that cannot be obtained in actual production. By optimizing the gear material and heat treatment process, predicting the change law of the carburized layer depth, hardness, and deformation of the gear, establishing the intelligent design and process optimization method of the gear carburizing and quenching process and forming a systematic research process, it has become one of the important research directions in the field of international heat treatment. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a simulation experimental method for surface heat treatment strengthening of steel gears in view of the deficiencies of the above-mentioned prior art. The method can analyze the influence of various process factors on gear performance during carburizing and quenching, and formulate a carburizing and quenching process optimization plan with hardness, deformation, and carburized layer depth as optimization targets. An optimization plan and evaluation method for the gear carburizing and quenching process are formed. The gear optimization process is efficiently obtained, the number of unnecessary gear trial production times is reduced, the production cost of the enterprise is reduced in production practice, and the economic benefits are improved.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is: a simulation experimental method for surface heat treatment strengthening of steel gears, characterized in that it includes the following steps:

[0005] S1. Build a calculation model:

[0006] S2. Establishing gear geometry model: Establishing FZG gear single tooth model, meshing the single tooth model, setting heat transfer and carburizing boundary conditions on the surface of the single tooth model, and fixing and constraining the single tooth model;

[0007] S3. Carburizing and quenching simulation is performed on the geometric model: carburizing and quenching simulation includes temperature field simulation, carbon concentration field simulation, structure transformation simulation, hardness simulation and deformation simulation;

[0008] S4. Carburizing and quenching test is performed on the gear entity;

[0009] S5. Compare and verify the simulation results through test results;

[0010] Preferably, the calculation model in S1 includes a carburizing concentration field model, a heat conduction model, a phase change kinetics model, a stress-strain model and a hardening rule model. Specifically, the relevant parameters of two 20MnCr5 steels with different chemical element contents are calculated by JMatpro software, and a nonlinear parameter database of the material carburizing and quenching process is established and supplemented.

[0011] Preferably, the carburizing concentration field model is constructed as follows: the carbon diffusion process is usually explained and analyzed using Fick's second law. Considering the diffusion rate of carbon in steel and the carbon concentration gradient, the diffusion coefficient and transfer coefficient of the carbon element are determined, and the control equation is as follows:

[0012]

[0013] Where C is the carbon content of the gear steel; t is the set carburizing time; x i is the carburizing position; D C is the carbon diffusion coefficient.

[0014] The carbon diffusion coefficient changes with the carbon concentration. The alloy composition of the material and the carburizing temperature will also affect the value of the carbon diffusion coefficient. The action equation is as follows:

[0015]

[0016] Among them, R C is the gas phase constant, which is 1.986 cal / mol / K; p is the influence factor of alloying elements.

[0017] The value of the alloying element influencing factor is related to the type and content of the alloying elements in the material. The calculation formula is as follows:

[0018] p=1+(0.15+0.033Si)Si-0.0365Mn-(0.13-5.5 e-3Cr)Cr+(0.03-0.03365Ni)Ni-(0.025-0.01Mo)Mo-(0.03-0.02Al)Al-(0.016+1.4 e-3 Cu)Cu-(0.22-0.01V)V

[0019] Among them, Si, Mn, Cr, etc. are the mass fractions of each element in gear steel.

[0020] It is necessary to clarify the initial conditions of carburizing. The initial conditions refer to the carbon concentration of the material before carburizing begins, and the carbon concentration is evenly distributed. The initial conditions are as follows:

[0021] C| t＝0 =C0

[0022] Among them, C0 is the initial carbon concentration in the gear steel, which is a constant.

[0023] Carbon atoms in the external carbon potential continuously diffuse into the gear through the gear surface. However, due to the limitation of carburizing time and diffusion time, the depth of the carburized layer is often within a certain range. Therefore, when the depth from the surface reaches a certain depth or even deeper, the carbon content at this position is still the original carbon content of the gear steel. The internal boundary condition is

[0024]

[0025] Among them, x max It is greater than the actual carburized layer depth desired to be obtained.

[0026] At the same time, during the carburizing process, the external carbon potential provides carbon atoms to penetrate and diffuse into the gear surface. This process is in a dynamic unbalanced state. The entire transfer process depends on the absorption process and chemical reaction process of the gear surface. The ability of carbon atoms to be transferred from the atmosphere to the gear is proportional to the difference in carbon mass fraction between the external atmosphere and the gear surface. The external boundary conditions are set as

[0027]

[0028] Among them, C w For the external atmosphere at x i Carbon potential at β C It is the transfer coefficient of carbon atoms from the atmosphere to the gear surface. Carburizing temperature, gas activity and gas pressure will have a certain impact on it.

[0029] Since the influence of carbon transfer coefficient is complex, only the influence of carburizing temperature is considered, and other values ​​are taken as constants. The calculation formula is:

[0030]

[0031] Among them, β0 is a constant related to material properties, and its value is 3.47e-3mm / s; E f is the reaction activation energy, which is 34 kJ / mol; T a is the carburizing temperature, when T a When the temperature is 1203K, the transfer coefficient β is obtained. C It is 1.1587e-4mm / s; R is the molar gas constant, which is 8.314J / (mol·K).

[0032] The specific method of constructing the heat conduction model is as follows:

[0033] Taking into account factors such as phase change latent heat, stress and strain, the heat transfer equation used is:

[0034]

[0035] Among them, ρ is the density of the mixed phase; c is the hot melt of the mixed phase; T is the temperature; σ is the stress; ε is the elastic strain; H is the enthalpy change density; l is the Ith component of the latent heat; ξ is the phase variable of the latent heat of phase change; k is the thermal conductivity; and x represents different positions.

[0036] Before carburizing and quenching, the temperature of each position of the gear is consistent, and the initial conditions are determined as follows:

[0037] T| t＝0 =T0

[0038] Among them, T0 is the known initial temperature and its value is a constant.

[0039] During the carburizing and quenching process, the temperature of the gear is different from that of the external medium, and convection and radiation phenomena occur continuously between the two to transfer heat. In order to more accurately simulate the changes in the temperature field of the gear during the carburizing and quenching process, it is necessary to set appropriate boundary conditions. The boundary conditions of heat conduction are based on the convection heat transfer coefficient between the gear and the contact medium and the medium temperature. The boundary conditions are set as follows:

[0040]

[0041] Among them, n i is the boundary of the gear; h T is the heat transfer coefficient between the gear and the medium; T M is the temperature of the external environment where the gear is located.

[0042] The steps for constructing the phase transition kinetic model are:

[0043] Using the Inoue model, the diffusion-type phase change volume fraction expression is as follows:

[0044]

[0045] Among them, ξ B / P is the volume fraction of generated bainite and pearlite; f1(T), f2(σ ij ), f3(C) are temperature T, stress σ ij , a function of carbon content C.

[0046] Using the Inoue model, the expression for the volume fraction of non-diffusion phase change is as follows:

[0047] ξ M =1-exp(δ1T+δ2(C-C0)+δ3σ m +δ4σ e +δ5))

[0048] Among them, ξ M is the volume fraction of generated martensite; σ m is the mean stress; σ e is the equivalent stress; δ1, δ2, δ3, δ4, δ5 are the test coefficients affected by temperature, carbon content, mean stress, equivalent stress, etc.

[0049] The stress-strain model is constructed as follows:

[0050] The deformation of gears after heat treatment is usually analyzed using an elastic-plastic material model, and the elastic-plastic problem is defined by determining the yield criterion, rheological rule, and hardening rule. The gear deformation during the entire carburizing and quenching process is the sum of the strain caused by temperature and phase change, so the deformation caused by each physical quantity at each stage needs to be superimposed. The heat treatment deformation expression is as follows:

[0051]

[0052] in, is the total strain rate; is the elastic strain rate; is the plastic strain rate; is the thermal strain rate; is the phase transformation strain rate; is the phase transformation plastic strain rate.

[0053] The calculation expressions of elastic strain and plastic strain are as follows

[0054]

[0055]

[0056] Where E is Young's modulus; v is Poisson's ratio; δ ij is the deviatoric stress; p is a function of stress, stress rate and strain history; ξ1 is a single phase; k j is the work hardening parameter; εp is the plastic strain.

[0057] The expressions of phase transformation strain and thermal strain are:

[0058]

[0059]

[0060] Among them, β I is the phase change coefficient of structural expansion caused by instantaneous phase change; α is a function of carbon content and structural volume fraction; I is the part where structural expansion occurs.

[0061] Phase change plasticity is mainly related to phase change type and temperature. The theoretical formula of phase change plasticity strain rate is as follows:

[0062]

[0063] Among them, k I is the phase transformation plasticity coefficient, which is an important parameter affecting heat treatment deformation.

[0064] The construction method of the hardening rule model is:

[0065] The hardness simulation solution is calculated by the organization type and volume fraction obtained after carburizing and quenching. The hardness value is estimated by superimposing each unit through the linear mixing principle. The hardness calculation model of gear steel after carburizing and quenching is as follows:

[0066]

[0067] Among them, ξ N is the volume fraction of different iron-carbon phases; γ N is the hardness of different iron-carbon phases; η K is the alloy composition; C K is the weight coefficient corresponding to the alloy composition.

[0068] The hardness calculation model of each iron phase structure is as follows:

[0069] Pearlite and cementite

[0070] γ F-P =42+223C+53Si+30Mn+12.6Ni+7Cr+19Mo+(10-19Si+4Ni+8Cr+130V)logV F-P

[0071] bainite

[0072] γ B=-323+185C+330Si+153Mn+65Ni+144Cr+191Mo+(89+53C-55Si-22Mn-10Ni-20Cr-33Mo)logVB

[0073] Martensite

[0074] γ M =127+949C+27Si+11Mn+8Ni+16Cr+21logV M

[0075] Among them, γ F-P , γ B , γ M are the hardness values ​​of ferrite, pearlite, bainite and martensite respectively; V F-P 、V B 、V M are the cooling rates of ferrite, pearlite, bainite and martensite respectively.

[0076] When C ≥ 0.5%, the above hardness formula of martensite is not applicable, so the following formula is used:

[0077]

[0078] After carburizing and quenching, most of the austenite structure on the gear surface has completed the transformation to martensite, but some austenite still exists. The existence of these residual austenite will also have a certain effect on the hardness of the gear. The calculation model of the hardness of the residual austenite structure is:

[0079]

[0080] Among them, γ RA is the hardness of retained austenite; RA is the volume fraction of retained austenite; in order to reflect the hardness value of gear steel in the simulation result cloud diagram, the hardness of retained austenite is taken as a negative value.

[0081] Preferably, in S2, cosmap software is used to establish a three-dimensional model of a single tooth of a FZG gear, the single tooth model is divided into hexahedral meshes, and the surface mesh is encrypted.

[0082] Preferably, the process design of the carburizing quenching simulation and the carburizing quenching test is as follows: the carburizing temperature is 930°C, the carburizing temperature is reached after heating for 40 minutes, carburizing for 130 minutes at a carbon potential of 1.1%, cooling to 855°C after carburizing, keeping warm for 30 minutes at a carbon potential of 0.75%, and finally rapidly cooling to room temperature in quenching oil.

[0083] Preferably, the temperature field simulation in S3 is specifically as follows: analyzing the dynamic temperature change process of a single tooth model made of two types of steel during carburizing and quenching, dividing the single tooth model along the middle plane in the tooth width direction, taking test points at the tooth top, pitch circle and tooth root of the dividing surface, and at the same time taking three test points along the pitch circle from the outside to the inside, obtaining the temperature-time curve and quenching temperature distribution cloud diagram of the test points and analyzing them.

[0084] The simulation of carbon concentration field is as follows: the carbon concentration is detected and analyzed at the pitch circle position of the single tooth model, and the dynamic change of carbon concentration in the strong carburization stage is analyzed at 6 points along the tooth surface of the single tooth model toward the gear core. At the same time, the carburized layer depth is detected and analyzed at the tooth top, pitch circle and tooth root respectively, and the carbon content-time curve and carbon content distribution curve are obtained and analyzed;

[0085] The specific structure transformation simulation is as follows: the microstructure distribution cloud map of the single tooth model after carburizing and quenching showing the distribution of martensite and bainite is obtained and analyzed, and the surface retained austenite content after carburizing and quenching is calculated using the Magee formula;

[0086] The specific hardness simulation is as follows: obtaining and analyzing the surface hardness distribution diagram after carburizing and quenching;

[0087] The deformation simulation is specifically as follows: obtaining and analyzing the deformation distribution diagram after carburizing and quenching.

[0088] Compared with the prior art, the present invention has the following advantages:

[0089] 1. The present invention provides a reference scheme and data support for the trial formulation and production of gear carburizing and quenching process schemes, avoiding the high trial and error costs and risks of heat treatment. The method is simple to operate, economical and reliable, and can be used as an effective guiding tool for actual experiments.

[0090] 2. The present invention carried out carburizing and quenching heat treatment tests on two types of 20MnCr5 steel FZG gears, built a gear carburizing and quenching process model, established a finite element model based on the metal-thermo-mechanical theory, and used COSMAP heat treatment software to simulate and analyze the gear temperature field, carbon concentration field, structure field, hardness field and deformation before and after carburizing and quenching, revealing the dynamic changes and distribution laws of each field of the gear during the carburizing and quenching process.

[0091] The present invention is further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0092] Figure 1 It is a schematic diagram of the overall process of the present invention.

[0093] Figure 2 It is a schematic diagram of the gear geometric model in the present invention.

[0094] Figure 3 It is the carburizing and quenching process curve diagram of the present invention.

[0095] Figure 4 It is a schematic diagram of the position for analyzing the dynamic temperature change in the present invention.

[0096] Figure 5 It is a temperature-time graph of the present invention.

[0097] Figure 6 It is the temperature distribution cloud diagram in the present invention.

[0098] Figure 7 It is a schematic diagram of the carbon concentration detection direction and point location in the present invention.

[0099] Figure 8 It is a carbon content-time curve diagram in the present invention.

[0100] Fig. 9 It is a schematic diagram of carbon content distribution in the present invention.

[0101] Fig.10 It is the microstructure distribution cloud diagram in the present invention.

[0102] Fig.11 It is a schematic diagram of the surface hardness distribution in the present invention.

[0103] Fig.12 It is a deformation distribution diagram in the present invention.

[0104] Fig.13 It is a deformation distribution curve diagram in the present invention.

[0105] Fig.14 It is a microstructure diagram after carburizing and quenching test verification in the present invention.

[0106] Fig.15 This is a microstructure diagram after carburizing and quenching test verification in the present invention.

[0107] Fig.16 It is a comparison diagram between simulation and experiment in the present invention.

[0108] Fig.17 It is a schematic diagram of the tooth direction error of the gear before and after carburizing and quenching in the present invention. DETAILED DESCRIPTION

[0109] like Figure 1 As shown, the present invention comprises the following steps:

[0110] S1. Build a calculation model:

[0111] The calculation model includes carburizing concentration field model, heat conduction model, phase transformation kinetics model, stress-strain model and hardening rule model. Specifically, the relevant parameters of 20MnCr5 steel with two different chemical element contents are calculated by JMatpro software, and a nonlinear parameter database of the material carburizing and quenching process is built and supplemented.

[0112] Preferably, the carburizing concentration field model is constructed as follows: the carbon diffusion process is usually explained and analyzed using Fick's second law. Considering the diffusion rate of carbon in steel and the carbon concentration gradient, the diffusion coefficient and transfer coefficient of the carbon element are determined, and the control equation is as follows:

[0113]

[0114] Where C is the carbon content of the gear steel; t is the set carburizing time; x i is the carburizing position; D C is the carbon diffusion coefficient.

[0115] The carbon diffusion coefficient changes with the carbon concentration. The alloy composition of the material and the carburizing temperature will also affect the value of the carbon diffusion coefficient. The action equation is as follows:

[0116]

[0117] Among them, R C is the gas phase constant, which is 1.986 cal / mol / K; p is the influence factor of alloying elements.

[0118] The value of the alloying element influencing factor is related to the type and content of the alloying elements in the material. The calculation formula is as follows:

[0119] p=1+(0.15+0.033Si)Si-0.0365Mn-(0.13-5.5 e-3 Cr)Cr

[0120] +(0.03-0.03365Ni)Ni-(0.025-0.01Mo)Mo-(0.03-0.02Al)Al

[0121] -(0.016+1.4 e-3 Cu)Cu-(0.22-0.01V)V

[0122] Among them, Si, Mn, Cr, etc. are the mass fractions of each element in gear steel.

[0123] It is necessary to clarify the initial conditions of carburizing. The initial conditions refer to the carbon concentration of the material before carburizing begins, and the carbon concentration is evenly distributed. The initial conditions are as follows:

[0124] C t＝0 =C0

[0125] Among them, C0 is the initial carbon concentration in the gear steel, which is a constant.

[0126] Carbon atoms in the external carbon potential continuously diffuse into the gear through the gear surface. However, due to the limitation of carburizing time and diffusion time, the depth of the carburized layer is often within a certain range. Therefore, when the depth from the surface reaches a certain depth or even deeper, the carbon content at this position is still the original carbon content of the gear steel. The internal boundary condition is

[0127]

[0128] Among them, x max It is greater than the actual carburized layer depth desired to be obtained.

[0129] At the same time, during the carburizing process, the external carbon potential provides carbon atoms to penetrate and diffuse into the gear surface. This process is in a dynamic unbalanced state. The entire transfer process depends on the absorption process and chemical reaction process of the gear surface. The ability of carbon atoms to be transferred from the atmosphere to the gear is proportional to the difference in carbon mass fraction between the external atmosphere and the gear surface. The external boundary conditions are set as

[0130]

[0131] Among them, C w For the external atmosphere at x i Carbon potential at β C It is the transfer coefficient of carbon atoms from the atmosphere to the gear surface. Carburizing temperature, gas activity and gas pressure will have a certain impact on it.

[0132] Since the influence of carbon transfer coefficient is complex, only the influence of carburizing temperature is considered, and other values ​​are taken as constants. The calculation formula is:

[0133]

[0134] Among them, β0 is a constant related to material properties, and its value is 3.47e-3mm / s; E f is the reaction activation energy, which is 34 kJ / mol; T a is the carburizing temperature, when T a When the temperature is 1203K, the transfer coefficient β is obtained. C It is 1.1587e-4mm / s; R is the molar gas constant, which is 8.314J / (mol·K).

[0135] The specific method of constructing the heat conduction model is as follows:

[0136] Taking into account factors such as phase change latent heat, stress and strain, the heat transfer equation used is:

[0137]

[0138] Among them, ρ is the density of the mixed phase; c is the hot melt of the mixed phase; T is the temperature; σ is the stress; ε is the elastic strain; H is the enthalpy change density; l is the Ith component of the latent heat; ξ is the phase variable of the latent heat of phase change; k is the thermal conductivity; and x represents different positions.

[0139] Before carburizing and quenching, the temperature of each position of the gear is consistent, and the initial conditions are determined as follows:

[0140] T| t＝0 =T0

[0141] Among them, T0 is the known initial temperature and its value is a constant.

[0142] During the carburizing and quenching process, the temperature of the gear is different from that of the external medium, and convection and radiation phenomena occur continuously between the two to transfer heat. In order to more accurately simulate the changes in the temperature field of the gear during the carburizing and quenching process, it is necessary to set appropriate boundary conditions. The boundary conditions of heat conduction are based on the convection heat transfer coefficient between the gear and the contact medium and the medium temperature. The boundary conditions are set as follows:

[0143]

[0144] Among them, n i is the boundary of the gear; h T is the heat transfer coefficient between the gear and the medium; T M is the temperature of the external environment where the gear is located.

[0145] The steps for constructing the phase transition kinetic model are:

[0146] Using the Inoue model, the diffusion-type phase change volume fraction expression is as follows:

[0147]

[0148] Among them, ξ B / P is the volume fraction of generated bainite and pearlite; f1(T), f2(σ ij ), f3(C) are temperature T, stress σ ij , a function of carbon content C.

[0149] Using the Inoue model, the expression for the volume fraction of non-diffusion phase change is as follows:

[0150] ξ M =1-exp(δ1T+δ2(C-C0)+δ3σ m +δ4σ e +δ5))

[0151] Among them, ξ Mis the volume fraction of generated martensite; σ m is the mean stress; σ e is the equivalent stress; δ1, δ2, δ3, δ4, δ5 are the test coefficients affected by temperature, carbon content, mean stress, equivalent stress, etc.

[0152] The stress-strain model is constructed as follows:

[0153] The deformation of gears after heat treatment is usually analyzed using an elastic-plastic material model, and the elastic-plastic problem is defined by determining the yield criterion, rheological rule, and hardening rule. The gear deformation during the entire carburizing and quenching process is the sum of the strain caused by temperature and phase change, so the deformation caused by each physical quantity at each stage needs to be superimposed. The heat treatment deformation expression is as follows:

[0154]

[0155] in, is the total strain rate; is the elastic strain rate; is the plastic strain rate; is the thermal strain rate; is the phase transformation strain rate; is the phase transformation plastic strain rate.

[0156] The calculation expressions of elastic strain and plastic strain are as follows

[0157]

[0158]

[0159] Where E is Young's modulus; v is Poisson's ratio; δ ij is the deviatoric stress; p is a function of stress, stress rate and strain history; ξ1 is a single phase; k j is the work hardening parameter; ε p is the plastic strain.

[0160] The expressions of phase transformation strain and thermal strain are:

[0161]

[0162]

[0163] Among them, β I is the phase change coefficient of structural expansion caused by instantaneous phase change; α is a function of carbon content and structural volume fraction; I is the part where structural expansion occurs.

[0164] Phase change plasticity is mainly related to phase change type and temperature. The theoretical formula of phase change plasticity strain rate is as follows:

[0165]

[0166] Among them, k I is the phase transformation plasticity coefficient, which is an important parameter affecting heat treatment deformation.

[0167] The construction method of the hardening rule model is:

[0168] The hardness simulation solution is calculated by the organization type and volume fraction obtained after carburizing and quenching. The hardness value is estimated by superimposing each unit through the linear mixing principle. The hardness calculation model of gear steel after carburizing and quenching is as follows:

[0169]

[0170] Among them, ξ N is the volume fraction of different iron-carbon phases; γ N is the hardness of different iron-carbon phases; η K is the alloy composition; C K is the weight coefficient corresponding to the alloy composition.

[0171] The hardness calculation model of each iron phase structure is as follows:

[0172] Pearlite and cementite

[0173] γ F-P =42+223C+53Si+30Mn+12.6Ni+7Cr+19Mo+(10-19Si+4Ni+8Cr+130V)logV F-P

[0174] bainite

[0175] γ B =-323+185C+330Si+153Mn+65Ni+144Cr+191Mo+(89+53C-55Si-22Mn-10Ni-20Cr-33Mo)logV B

[0176] Martensite

[0177] γ M =127+949C+27Si+11Mn+8Ni+16Cr+21logV M

[0178] Among them, γ F-P , γ B , γ M are the hardness values ​​of ferrite, pearlite, bainite and martensite respectively; V F-P 、V B 、V Mare the cooling rates of ferrite, pearlite, bainite and martensite respectively.

[0179] When C ≥ 0.5%, the above hardness formula of martensite is not applicable, so the following formula is used:

[0180]

[0181] After carburizing and quenching, most of the austenite structure on the gear surface has completed the transformation to martensite, but some austenite still exists. The existence of these residual austenite will also have a certain effect on the hardness of the gear. The calculation model of the hardness of the residual austenite structure is:

[0182]

[0183] Among them, γ RA is the hardness of retained austenite; RA is the volume fraction of retained austenite; in order to reflect the hardness value of gear steel in the simulation result cloud diagram, the hardness of retained austenite is taken as a negative value.

[0184] S2. Establish the gear geometry model: Use the 2Hexahedral unit type in the cosmap software to divide the single gear model into hexahedral meshes. In order to improve the simulation accuracy of the tooth surface hardness, the gear surface mesh is encrypted. The FZG gear single tooth 3D model is as follows: Figure 2 As shown in (a), the three-dimensional model is divided into 8856 grid units and 10227 nodes. During the carburizing and quenching process, there will be a temperature difference and carbon concentration difference between the gear surface and the outside world. The temperature and carbon atoms are continuously transferred between the outside world and the inside of the gear, and the transfer process at different positions on the gear will also be different. Figure 2 As shown in (b), heat transfer and carburizing boundary conditions are set at different locations on the surface of the gear model. The carburizing and quenching process of the gear will cause phase change and strain, causing gear deformation. The three-dimensional model of the single gear tooth needs to be fixed and constrained. The constraint conditions are as follows: Figure 2 As shown in (c), points A1 and A2 fix the rotation and movement of the model in the yz direction, and points B1 and B2 fix the rotation and displacement of the model in the xz direction;

[0185] S3. Carburizing and quenching simulation of the geometric model: Figure 3 As shown, the process design of the carburizing quenching simulation and the carburizing quenching test described in this embodiment is as follows: the carburizing temperature is 930°C, the carburizing temperature is reached after heating for 40 minutes, carburizing for 130 minutes at a carbon potential of 1.1%, cooling to 855°C after carburizing, keeping warm for 30 minutes at a carbon potential of 0.75%, and finally quickly cooling to room temperature in quenching oil.

[0186] Carburizing and quenching simulation includes temperature field simulation, carbon concentration field simulation, structure transformation simulation, hardness simulation and deformation simulation.

[0187] The specific temperature field simulation is as follows: analyze the dynamic temperature change process of two gears made of 20MnCr5-A and 20MnCr5-B steel during carburizing and quenching, and divide the middle plane of the single gear along the tooth width direction. Take test points at the tooth top, pitch circle and tooth root of the split position respectively. At the same time, take three test points from the outside to the inside along the pitch circle position for analysis. The positions of all the points are as follows: Figure 4 As shown. a1, b, c are the points from the surface to the core of the gear pitch circle, and a2 and a3 are the tooth top and tooth root positions, respectively.

[0188] The temperature field variation curves of gears A and B during the quenching process are shown in Figure 5 As shown. It can be seen that the temperature change trends of the two gears are similar, and the temperatures of points a1, b, and c increase successively. After quenching for 4s, the temperature difference between the inside and outside of the gear is 319°C, which shows that the closer to the gear surface, the faster the cooling speed. When the tooth surface temperature drops to 600°C, the cooling rate of the gear core increases significantly. During the quenching process, the tooth surface temperature drops rapidly, and it reaches the temperature of each phase change point first. Phase change occurs first and generates phase change latent heat. Part of the heat generated is transferred to the core, further delaying the decrease in the temperature of the tooth core. During the entire quenching process, the temperature difference between the tooth surface and the tooth core undergoes a process of rapid increase and slow decrease, and finally the temperature of the gear surface and the core tends to be consistent.

[0189] Figure 6 This is the temperature distribution cloud diagram of the gear when it is quenched for 4s. In the initial stage of quenching, the surface cooling rate at point a2 of the tooth top is faster than that at point a1 of the pitch circle, and the surface cooling rate at point a3 of the tooth root is slower than that at point a1 of the pitch circle. This is because the pitch circle and the tooth root are closer to the core of the gear than the tooth top, and the maximum temperature difference between different positions of the tooth surface of the two gears during quenching is 152°C. When the tooth surface temperature is about 650°C, the cooling rate of the tooth surface and the tooth root surface is greater than that of the tooth top, and the heat transfer rate of the gear surface reaches its peak at this time. When the tooth surface temperature continues to drop, the cooling rate slowly decreases due to the decrease in the heat transfer coefficient of the quenching medium and the latent heat of phase change.

[0190] The simulation of carbon concentration field is as follows: carburizing process and diffusion process affect the distribution of carbon content on the tooth surface. The carbon concentration field is detected and analyzed at the pitch circle position of a single tooth, and the dynamic change of carbon concentration in the strong carburizing stage is analyzed at 6 points (d1, d2, d3, d4, d5, d6) along the tooth surface to the gear core. At the same time, the depth of the carburized layer after carburizing and quenching is detected and analyzed at the tooth top, pitch circle and tooth root. The detection direction and point location are as follows: Figure 7 As shown, the arrow marks the measurement direction.

[0191] The carbon content distribution of 20MnCr5-A and 20MnCr5-B steel gears after carburizing and quenching is as follows: Figure 8 As shown. In the initial stage, the carbon concentration on the tooth surface begins to rise rapidly, reaching a peak of about 1.1% when approaching the external carbon concentration. From the trend of carbon concentration changes at different points, it can be seen that from point d1 to point d6, the carbon concentration peak decreases successively, the lag time of carbon element transfer from the gear surface to the core gradually increases, and the carbon concentration peak decreases continuously. At point d6, the carbon element is almost unaffected by the external carbon potential. Within the limited strong penetration time, when carbon atoms diffuse into the interior of the gear, a large number of carbon atoms will be absorbed by the surface of the gear, and only a small number of carbon atoms continue to diffuse inward, resulting in the carbon concentration difference being smaller and the carbon element transfer rate being smaller as it gets closer to the inside of the gear.

[0192] The carbon content distribution curve of the gear at different positions is as follows: Fig. 9 As shown. It can be seen that the carbon content on the surface of the two gears of 20MnCr5-A and 20MnCr5-B steel is about 0.75%. In this paper, the carbon content of 0.40% is used as the limit carbon content of the carburized layer depth. The carburized layer depths of the two gears of 20MnCr5-A and 20MnCr5-B steel at the pitch circle are 0.70mm and 0.75mm respectively, and the carburized layer depth of the 20MnCr5-B steel gear is deeper than that of the 20MnCr5-A steel gear. Because the Mn and Cr elements in the 20MnCr5-B steel gear have higher content, it is easier to combine with carbon atoms, and the gear surface has a deeper carburized layer. The steel grades of the two gears are both 20MnCr5 steel, and the types and contents of the elements contained are similar, and the diffusion coefficients of the carbon element are slightly different, so the carburized layer depths of the two gears are also similar. The carburized layer depth at the top of the two gears is the deepest, and the carburized layer depths at the pitch circle and the root are similar. Due to the sharp angle effect at the tooth top, there are multiple directions of high carbon potential sources diffusing inward during the diffusion process. The depth of the carburized layer at the tooth top is significantly higher than that at the pitch circle and tooth root of the gear. The core carbon content of the 20MnCr5-A and 20MnCr5-B steel gears is 0.20% and 0.18% respectively. The trend line shows that at the pitch circle and tooth root of the gear, when the depth from the tooth surface is ≥0.4mm, the carbon content in the organization is almost unaffected by the external carbon potential.

[0193] The specific structure transformation simulation is as follows: the martensite and bainite structures in the material can improve hardness and stiffness. Compared with bainite structure, martensite structure has higher strength and hardness. The content and distribution of martensite structure in the tooth surface is one of the important factors affecting the surface hardness. During the carburizing and quenching process, more austenite structure needs to be transformed into martensite, thereby improving the hardness and wear resistance of the gear surface.

[0194] like Fig.10The following is a cloud diagram of the microstructure distribution of 20MnCr5-A and 20MnCr5-B steel gears after carburizing and quenching. After quenching, a large amount of martensite structure is generated on the surface of both gears, and the martensite content gradually decreases from the surface to the core of the gear. The highest martensite volume fraction on the surface of the 20MnCr5-A steel gear is 90.3%, including 90.3% at the tooth top, 89.6% at the pitch circle, and 87.4% at the tooth root; the highest martensite volume fraction on the surface of the 20MnCr5-B steel gear is 94.6%, slightly higher than that of the 20MnCr5-A steel gear. The martensite volume fraction at the tooth top of the 20MnCr5-B steel gear is 94.6%, 93.6% at the pitch circle, and 90.1% at the tooth root. This is because the tooth top is far away from the core, and the temperature of the tooth top decreases faster during quenching. The tooth surface and tooth root are closer to the core, and the temperature drops more slowly during quenching, so the martensite content at the tooth top is higher. Due to the high carbon concentration on the gear surface, the Ms (starting temperature of martensite transformation) point decreases, and some residual austenite structure remains after quenching. The volume fractions of retained austenite on the surface of 20MnCr5-A and 20MnCr5-B steel gears are 3.0% and 4.0%, respectively, and the volume fractions of martensite in the core are 56.3% and 36.7%, respectively. In order to clarify the content of retained austenite on the gear surface after carburizing and quenching, the Magee formula is used for calculation, and the calculation results are compared and analyzed with the simulation results. The formula is as follows

[0195] ξ RA =exp[-α(M s -T o )]

[0196] Among them, α is a constant, with a value of 0.011; Ms is the temperature at which austenite begins to transform into martensite; To is the quenching oil temperature, with a value of 100℃.

[0197] Combined with the CCT curve of the material, it can be seen that the Ms value of the 20MnCr5-A gear is 388.1℃, and the Ms value of the 20MnCr5-B gear is 397.2℃. Calculation shows that the retained austenite content on the surface of 20MnCr5-A and 20MnCr5-B steel gears is 4.2% and 3.8%, respectively, which is close to the simulation value.

[0198] Depend on Fig.10 It can be seen that the volume fraction of bainite on the tooth surface of 20MnCr5-A and 20MnCr5-B steel gears is 7% and 1% respectively, and the maximum transformation of bainite in the core of the two gears is 26% and 45% respectively. The transformation of bainite structure at the surface of the two gears is small, while there is more bainite transformation in the core of the gear.

[0199] The specific hardness simulation is as follows: The hardness simulation results of the gear tooth surface of 20MnCr5-A and 20MnCr5-B steel are as follows: Fig.11As shown. It can be seen that the gear surface hardness decreases along the tooth top to the pitch circle and the tooth root. The hardness range of the 20MnCr5-A steel gear tooth surface is 674-676HV, and the hardness range of the 20MnCr5-B steel gear tooth surface is 683-684HV. The 20MnCr5-B steel gear surface has higher hardness everywhere, and the tooth surface hardness fluctuation is smaller. The hardness is mainly related to the volume fraction of the martensite structure on the tooth surface and the carbon concentration in the structure.

[0200] The deformation simulation is as follows: Fig.12 The figure shows the simulated gear deformation distribution. Fig.13 The deformation curves of different positions of the tooth surface, such as the tooth top, pitch circle, and tooth root. The maximum deformations of 20MnCr5-A and 20MnCr5-B steel gears are 43.3μm and 32.7μm, respectively, and the minimum deformations are 0.6μm and 0.4μm, respectively. The average deformations at the tooth top are 26.4μm and 29.3μm, respectively, and the maximum deformations are 30.4μm and 31.1μm, respectively. The average deformations at the pitch circle are 17.3μm and 15.9μm, respectively, and the maximum deformations are 23.3μm and 20.5μm, respectively. The average deformations at the tooth root are 17.6μm and 15.2μm, respectively, and the maximum deformations are 24.5μm and 20.5μm, respectively. The distribution law of the deformation of the two gears is basically the same. The deformation at the tooth top on the tooth surface is the largest, and the deformation gradually decreases from the tooth top to the tooth root, and the deformation at the pitch circle position is similar to that at the tooth root position. Along the tooth width direction, the deformation at each position on the tooth surface shows a trend of first decreasing and then increasing, and the deformation at the two end faces of the gear is the largest. From the temperature field cloud map of the quenching process, it can be seen that when the quenching process lasts for 4 seconds, along the tooth width direction, the temperature near the end face is lower and the cooling rate is higher. The temperature difference causes uneven structural stress and thermal stress, which eventually causes the gear to deform.

[0201] S4. Carry out carburizing and quenching tests on the gear entity, and compare and verify the simulation results through the test results.

[0202] Comparison of organizational changes: Figure 14-15 Figure 2 shows the microstructure distribution of two gears made of 20MnCr5-A and 20MnCr5-B steel after carburizing and quenching tests. Fig.14 It shows that the surface structure of 20MnCr5-A and 20MnCr5-B steel gears after carburizing and quenching is composed of martensite, carbide and retained austenite. Fig.15It can be seen that there are more fine needle-shaped martensite (M) in the surface structure of the two gears, and there is a small amount of retained austenite (Ar). The surface structure of 20MnCr5-B steel gears is finer, with more martensite structure and less retained austenite than that of 20MnCr5-A steel gears, and the size, state and distribution of carbides are significantly improved. These are consistent with the results of organizational simulation. The experimental measurement of the surface hardness of 20MnCr5-A and 20MnCr5-B steel gears is 690HV and 701HV respectively.

[0203] From the tooth surface to the core, the organization gradually transitions from high-carbon martensite to low-carbon martensite. The cores of 20MnCr5-A and 20MnCr5-B steel gears are both lath martensite and bainite distributed along the grain boundaries. The core martensite organizations of the two are similar, which is consistent with the simulation results. The thickness of the lath martensite in the core organization will affect its hardness and strength. The core organizations of the two gears are similar, indicating that the hardness of the two is relatively close. After measurement, the core hardness of gears A and B is 419HV and 418HV respectively, which are basically the same.

[0204] Hardness change comparison: Fig.16 The figure shows the comparison between the simulation value and the test result of the carburized layer of the gear after carburizing and quenching. Fig.16 (a) is the surface hardness gradient curve of the pitch circle position of 20MnCr5-A and 20MnCr5-B steel gears. It can be seen that the surface hardness of the 20MnCr5-A steel gear is 690HV, and the surface hardness of the 20MnCr5-B steel gear is 701HV. The hardness of both gears decreases with the increase of depth. This is because a large amount of martensite is formed on the surface of the gear, and the surface layer has a high carbon content. The volume fraction and carbon content of martensite both show a decreasing characteristic from the outside to the inside, resulting in high hardness on the tooth surface and low hardness on the tooth core. At the same depth, the hardness of 20MnCr5-B steel gears is generally higher than that of 20MnCr5-A steel gears. The carburized layer depths of 20MnCr5-A and 20MnCr5-B steel gears are 0.69mm and 0.84mm, respectively, and the carburized layer depth of 20MnCr5-B steel gears is deeper.

[0205] Fig.16 (b) is the depth of the carburized layer after carburizing and quenching of the gear. There is a certain difference between the simulated value and the measured value of the carburized layer depth. The maximum difference between the simulated value and the experimental value of the carburized layer depth of 20MnCr5-A and 20MnCr5-B steel gears is 0.01mm and 0.09mm respectively, and the maximum simulation error is ≤0.09mm. This is because the measured value of the carburized layer only detects a small area, while the simulated value is the average value of the tooth surface area.

[0206] The comparison between the simulated and experimental values ​​of the tooth surface hardness is shown in Table 1. The maximum differences between the simulated and experimental values ​​of the surface hardness of 20MnCr5-A and 20MnCr5-B steel gears are 16HV and 18HV, respectively. The simulation errors are 2.03-2.32% and 2.43-2.57%, respectively, within 3%, which verifies the accuracy of the simulation analysis.

[0207]

[0208] Table 1 Comparison of tooth surface hardness simulation value and test value

[0209] Deformation comparison: In order to verify the accuracy of the gear deformation simulation model, the gear accuracy test was performed to obtain the gear tooth error values ​​before and after heat treatment. The tooth error is the end face distance between the two smallest designed tooth lines on the gear pitch circle and within the effective range of the tooth width, including the actual tooth shape, and is expressed as F. β Indicates. β1 is the measurement result before heat treatment, F β2 is the measurement result after heat treatment. The difference between the two is the deformation before and after carburizing and quenching.

[0210] Tables 2 and 3 are the deformation measurement results of 20MnCr5-A and 20MnCr5-B steel gears, respectively. To ensure the accuracy of the results, four symmetrical single teeth on the gear were taken for measurement and analysis. The average deformation of the 20MnCr5-A steel gear near the pitch circle is 14.5μm, and the maximum deformation is 25μm. The average deformation of the 20MnCr5-B steel gear near the pitch circle is 18.9μm, and the maximum deformation is 24.5μm. During the gear processing process, the maximum deformation of the gear will affect the tooth profile and tooth direction accuracy of the gear after grinding. Therefore, the 20MnCr5-B steel gear has a stronger ability to control distortion.

[0211]

[0212] Table 2 Deformation of gear tooth surface after carburizing and quenching

[0213]

[0214] Table 3 Deformation of gear tooth surface after carburizing and quenching

[0215] The simulated and measured values ​​of the gear pitch circle position are shown in Table 4. The simulation errors of the average and maximum deformation values ​​of the 20MnCr5-A steel gear model are 2.8μm and 0.5μm, respectively. The simulation errors of the average and maximum deformation values ​​of the 20MnCr5-B steel gear model are 3μm and 4μm, respectively. The difference between the simulated and experimental values ​​is small, which verifies the accuracy of the model simulation analysis.

[0216]

[0217] Table 4 Comparison between simulation value and test value of pitch circle position deformation of tooth surface

[0218] Therefore, it can be concluded that the 20MnCr5-B steel gear has a higher hardness after carburizing and quenching than the 20MnCr5-A steel gear, a deeper carburized layer, and a relatively smaller maximum deformation of the tooth surface. After carburizing and quenching, the 20MnCr5-B steel has better performance.

[0219] In summary, the following conclusions can be drawn:

[0220] (1) During the carburizing and quenching process, the initial quenching cooling rate, carbon concentration, martensite volume fraction, hardness value, and deformation at the tooth surface position all reached their maximum values ​​at the tooth top position, and then decreased from the tooth top to the pitch circle and tooth root.

[0221] (2) During the carburizing and quenching process, different positions of the gear have different temperature values ​​and cooling rates, which will produce different structural stresses and thermal stresses, causing gear deformation. The gear deformation phenomenon shows that along the tooth width direction, the deformation amount near the end face is always greater than the tooth core.

[0222] (3) After carburizing and quenching, the maximum carbon content of the tooth surfaces of both gears reached 0.75%, and the depth of the carburized layer was about 0.7mm. The surface martensite transformation of the 20MnCr5-B steel gear was greater, and the tooth surface hardness was higher. The maximum deformation of the 20MnCr5-B steel gear at the pitch circle was smaller.

[0223] (4) Comparison and analysis between simulation values ​​and test values ​​show that the simulation error of gear hardness value is ≤3%, the simulation error of carburizing layer depth is ≤0.09mm, and the simulation error of deformation is ≤4μm. The simulation accuracy of the model is high, indicating that the model can be used as an effective guiding tool for actual experiments, providing prerequisites for subsequent heat treatment process factor analysis and gear heat treatment process optimization design based on tooth surface hardness, carburizing layer depth and deformation.

[0224] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent change made to the above embodiment according to the technical essence of the invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A simulation experimental method for heat treatment strengthening of steel gear surface, characterized in that: The following steps are involved: S1. Build a calculation model: S2. Establishing gear geometry model: Establishing FZG gear single tooth model, meshing the single tooth model, setting heat transfer and carburizing boundary conditions on the surface of the single tooth model, and fixing and constraining the single tooth model; S3. Carburizing and quenching simulation is performed on the geometric model: carburizing and quenching simulation includes temperature field simulation, carbon concentration field simulation, structure transformation simulation, hardness simulation and deformation simulation; S4. Carburizing and quenching test is performed on the gear entity; S5. Compare and verify simulation results with test results The temperature field simulation in S3 is specifically as follows: analyzing the dynamic temperature change process of the single tooth model made of two kinds of steel during the carburizing and quenching process, dividing the single tooth model along the middle plane of the tooth width direction, taking test points at the tooth top, pitch circle and tooth root of the dividing surface, and taking three test points along the pitch circle from the outside to the inside, obtaining the temperature-time curve and quenching temperature distribution cloud map of the test points and analyzing them; The carbon concentration field simulation in S3 is specifically as follows: carbon concentration is detected and analyzed at the pitch circle position of the single tooth model, and the dynamic change analysis of carbon concentration in the strong carburization stage is performed on the six points of the single tooth model along the tooth surface toward the gear core. At the same time, the carburized layer depth is detected and analyzed at the tooth top, pitch circle and tooth root respectively, and the carbon content-time curve and carbon content distribution curve are obtained and analyzed; The structural transformation simulation in S3 is specifically as follows: obtaining and analyzing the microstructure distribution cloud map of the single tooth model after carburizing and quenching showing the distribution of martensite and bainite, and calculating and clarifying the surface retained austenite content after carburizing and quenching using the Magee formula; The hardness simulation specifically includes: obtaining and analyzing the surface hardness distribution diagram after carburizing and quenching; The deformation simulation specifically includes: obtaining and analyzing a deformation distribution diagram after carburizing and quenching.

2. The simulation experimental method for heat treatment strengthening of steel gear surface according to claim 1, characterized in that: The calculation model in S1 includes a carburizing concentration field model, a heat conduction model, a phase transformation kinetics model, a stress-strain model and a hardening rule model. Specifically, the relevant parameters of two 20MnCr5 steels with different chemical element contents are calculated by JMatpro software, and a nonlinear parameter database of the material carburizing and quenching process is established and supplemented.

3. The simulation experimental method for heat treatment strengthening of steel gear surface according to claim 1, characterized in that: In the S2, cosmap software is used to establish a three-dimensional model of a single tooth of a FZG gear, the single tooth model is divided into hexahedral meshes, and the surface mesh is encrypted.

4. The simulation experimental method for heat treatment strengthening of steel gear surface according to claim 1, characterized in that: The process design of the carburizing quenching simulation and the carburizing quenching test is as follows: the carburizing temperature is 930°C, the carburizing temperature is reached after heating for 40 minutes, carburizing for 130 minutes at a carbon potential of 1.1%, cooling to 855°C after carburizing, keeping warm for 30 minutes at a carbon potential of 0.75%, and finally rapidly cooling to room temperature in quenching oil.

Images