Finite element analysis method for low temperature thermo-chemical carburizing / nitriding case diffusion and stress distribution

Abstract

The application provides a low-temperature thermo-chemical carburizing / nitriding layer diffusion and stress distribution finite element analysis method, and belongs to the technical field of material mechanical behavior analysis and finite element simulation. The method comprises the following steps: S1. constructing a three-dimensional model and performing mesh division and refinement; S2. constructing a mass transfer and heat transfer parameter definition calculation model related to carbon / nitrogen concentration; S3. obtaining material parameters related to concentration, wherein the material parameters comprise: the elastic modulus, yield strength and hardening index of the carburized / nitrided layer material under different carburizing / nitriding concentration conditions; the expansion coefficient under different carburizing / nitriding concentration conditions; S4. inputting the material parameters in S3 into the finite element model; and S5. obtaining carbon concentration distribution and residual stress distribution in the three-dimensional component through numerical simulation, so that the distribution of the carbon / nitrogen content in the carburized / nitrided layer can be accurately calculated, the distribution result is consistent with the test result, and the residual stress distribution result is consistent with the test measured value.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of material mechanical behavior analysis and finite element simulation, and particularly relates to a finite element analysis method for low-temperature thermo-chemical carburizing / nitriding layer diffusion and stress distribution. BACKGROUND

[0002] Low-temperature thermo-chemical surface strengthening technology is a surface modification technology that penetrates carbon, nitrogen or carbon-nitrogen co-penetrating elements into the metal surface below the traditional phase transition temperature. The technology forms a carburizing / nitriding layer with a thickness usually not exceeding 50 microns on the metal surface through a diffusion process. The technology can significantly improve the hardness and wear resistance of the surface while maintaining the toughness of the matrix. The high hardness of the carburizing layer is derived from the combination of alloying elements and penetrating elements, and the high compressive residual stress is caused by the volume expansion and stress field reconstruction due to the diffusion of the carburizing layer. The high compressive residual stress of the carburizing layer can significantly improve the fatigue performance of the material surface, so that the technology is widely used in precision mechanical parts, medical devices, aerospace components and transmission gears in the automotive industry.

[0003] However, the current modeling method for the behavior of the carburizing layer in the low-temperature thermo-chemical surface strengthening process is mainly limited to two-dimensional scale. Peng et al. established a carbon diffusion method for low-temperature thermo-chemical carburizing layer in the article "Experimental and numerical analysis of residual stress in carbon-stabilized expanded austenite". This two-dimensional modeling ignores the stress coupling between the matrix and the carburizing layer, resulting in low prediction accuracy of the residual stress in the carburizing layer. Since the high compressive residual stress of the carburizing layer is crucial to the improvement of fatigue performance, accurate simulation of the distribution of residual stress is of great significance to the optimization of process and the improvement of performance. Traditional two-dimensional modeling cannot fully describe the three-dimensional stress distribution characteristics of complex components, limiting its application to complex geometric components.

[0004] Therefore, a three-dimensional modeling is needed, which can simulate and predict the distribution characteristics of residual stress in the carburizing layer to accurately predict the mechanical behavior of the carburizing / nitriding layer material and provide a theoretical basis and technical support for the performance optimization and process improvement of complex geometric components. SUMMARY

[0005] Therefore, the application aims to provide a finite element analysis method for low-temperature thermo-chemical carburizing / nitriding layer diffusion and stress distribution. The finite element analysis method for low-temperature thermo-chemical carburizing / nitriding layer diffusion and stress distribution established by the application can accurately calculate the distribution of carbon / nitrogen content in the carburizing layer, and the distribution results are consistent with the test results and the residual stress distribution results are consistent with the test measured values.

[0006] In order to achieve the above object, the present application provides the following technical solutions.

[0007] The present application provides a finite element analysis method for low-temperature thermo-chemical carburizing / nitriding layer diffusion and stress distribution, comprising the following steps:

[0008] S1. Construct a three-dimensional model and perform mesh division and refinement, the specific method being:

[0009] In the three-dimensional model, a mesh refinement strategy in the depth direction of the carburizing / nitriding layer is adopted, high-density meshes are divided in the carburizing / nitriding layer region to ensure calculation accuracy; in the matrix region and the part far from the carburizing layer, larger meshes are adopted to reduce the overall calculation amount and improve modeling efficiency;

[0010] S2. Construct a mass transfer and heat transfer parameter definition calculation model related to the carbon / nitrogen concentration:

[0011] Based on the UMATHT module, a calculation model of the non-steady-state diffusion coefficient of carbon / nitrogen is defined to simulate the influence of the concentration gradient on the diffusion behavior, and the diffusion coefficient is dynamically associated with the concentration field;

[0012] S3. Obtain material parameters related to the concentration, including:

[0013] Elastic modulus, yield strength and hardening index of the carburizing layer material under different carburizing / nitriding concentrations;

[0014] Expansion coefficient under different carburizing / nitriding concentrations;

[0015] S4. Input the material parameters in S3 into the finite element model to fit the concentration-expansion coefficient relationship model; construct a dynamic association database of the concentration field and the material parameters for material performance definition in the finite element analysis;

[0016] S5. Obtain the carbon concentration distribution and residual stress distribution in the three-dimensional component through numerical simulation; compare the simulation results with the concentration distribution and residual stress test data obtained by experiments to verify the accuracy of the concentration distribution and residual stress distribution results.

[0017] As a preferred, the three-dimensional model is established in the ABAQUS finite element software.

[0018] As a preferred, the minimum size of the high-density average mesh divided in the carburizing / nitriding layer region in S1 is 0.001-0.005 mm, and the maximum mesh size is not more than 1 / 10 of the total depth of the carburizing layer; an incremental mesh size division mode is set from the carburizing layer region to the matrix region, specifically, the minimum mesh size near the carburizing layer region is 0.001-0.005 mm, and the mesh size near the matrix region is 0.1-0.5 mm.

[0019] As preferred, the calculation model in S2 is:

[0020]

[0021] wherein D represents the diffusion coefficient, D0 represents the diffusion constant, exp() represents the exponential term, k represents the proportional coefficient, C represents the current concentration, C max represents the maximum concentration.

[0022] As preferred, the elastic modulus, yield strength, hardening index in S3 are obtained by nanoindentation experiment;

[0023] The expansion coefficient under different concentrations is obtained by X-ray diffractometer.

[0024] As preferred, the concentration distribution in the carburized / nitrided layer is obtained by electron probe micro analyzer in S5, the residual stress in the carburized / nitrided layer is measured by X-ray diffraction technology; the obtained experimental data is compared and analyzed with the numerical simulation results.

[0025] At least the following beneficial technical effects are contained:

[0026] (1) The low-temperature thermo-chemical carburized / nitrided layer diffusion and stress distribution finite element analysis method established by using the present application can accurately calculate the distribution of carbon / nitrogen content in the carburized layer, and the distribution results are consistent with the experimental results.

[0027] (2) The low-temperature thermo-chemical carburized / nitrided layer finite element model established by the present application can obtain the compression residual stress distribution characteristics in very thin carburized layer in a large-size finite element model, and the residual stress distribution results are consistent with the experimental measurement values. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is a flow chart of the finite element analysis method for low-temperature thermo-chemical carburized / nitrided layer diffusion and stress distribution;

[0029] Figure 2 It is a finite element model diagram for grid optimization of the carburized layer in step S1 of Example 1;

[0030] Figure 3 It is a comparison diagram of the carbon concentration distribution results obtained in the carburized layer in step S2 of Example 1 and the experimental carbon concentration results;

[0031] Figure 4 It is a comparison diagram of the residual stress distribution results obtained in the carburized layer in step S5 of Example 1 and the experimental residual stress results. DETAILED DESCRIPTION

[0032] The following detailed description of various exemplary embodiments of the application will not be considered limiting of the application, but rather a description of certain aspects, features, and embodiments of the application.

[0033] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Additionally, for a range of values of a parameter, unless otherwise stated, each intervening value of the parameter is also specifically included within the scope of the present application. The intervening values of the parameter are combined with a stated value of the parameter in range form. These are only exemplary of the various preferred embodiments and are not intended to be limiting of the application. Additionally, other variations that are within the spirit of the application will occur to those skilled in the art upon consideration of this description. Accordingly, the application is not limited to the specific embodiments described herein, but is intended to cover all modifications and variations of this description which are within the scope of the application.

[0034] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the preferred methods and materials are described. All patents, patent applications, publications, and descriptions mentioned herein are incorporated by reference to the extent allowed by law. Nothing herein is to be construed as an admission that the application is not entitled to antedate such disclosure by virtue of prior application.

[0035] Various modifications and variations of the described embodiments of the application will be apparent to those skilled in the art without departing from the scope or spirit of the application. Although the application has been described in connection with specific preferred embodiments, it will be evident for a person skilled in the art that many other variations, modifications and applications exist. The specification and examples given are intended as illustrative only and not limiting of the application.

[0036] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having", "contains", "containing", or variations thereof, are intended to be open-ended and do not limit the application to the recited elements.

[0037] As used herein, the terms "room temperature", "ambient temperature" refer to 25 ± 2 °C, unless otherwise specified.

[0038] The raw materials or instruments used in the following examples of the present application are commercially available, unless otherwise specified.

[0039] The austenitic stainless steel used in the embodiments of the present application is 316L, with a size of 10 mm x 10 mm x 10 mm cubic piece, and the low-temperature thermo-chemical surface treatment process used is low-temperature gas carburizing.

[0040] Example 1

[0041] The present embodiment provides a low-temperature thermo-chemical carburizing / nitriding layer diffusion and stress distribution finite element analysis method, comprising the following steps:

[0042] S1. A three-dimensional model of 10mmx10mmx10mm is established in ABAQUS finite element software, wherein the average distribution is adopted for the grid size of the carburized layer, and the grid size of the carburized layer to the substrate direction adopts the gradient increasing strategy. Specifically, the grid size in the depth direction of the carburized layer is 0.002; the gradient increasing grid size is set between the carburized layer to the substrate region, specifically, the grid size close to the carburized layer region is 0.002mm, and the grid size close to the substrate region is 0.5mm, and the grid division result is shown in Figure 2 .

[0043] S2. On the basis of step S1, the non-steady state diffusion coefficient of carbon diffusion is defined based on the UMATHT module in the ABAQUS software to simulate the influence of carbon concentration gradient on the diffusion behavior, and the non-steady state diffusion coefficient is defined by using the following calculation model:

[0044]

[0045] In the formula, D represents the diffusion coefficient, D0 represents the diffusion constant, exp() represents the exponential term, k represents the proportional coefficient, C represents the current carbon concentration, C max represents the maximum carbon concentration.

[0046] S3. The elastic modulus, yield strength and hardening index of the austenitic stainless steel under different concentrations of carburizing are obtained by nanoindentation experiment.

[0047] The expansion coefficient of the austenitic stainless steel under different concentrations of carburizing is obtained by X-ray diffractometer.

[0048] S4. The material parameters in S3 are input into the finite element model to obtain the concentration-expansion coefficient relationship model; a dynamic correlation database of concentration field and material parameters is constructed for the definition of material performance in finite element analysis;

[0049] S5. The established finite element model of low-temperature thermo-chemical carburized layer is used to predict the carbon concentration distribution and residual stress distribution in the carburized layer; the simulation results are compared with the carbon concentration distribution (see Figure 3 ) and residual stress (see Figure 4 ) test data obtained by carburizing experiment to verify the accuracy of the concentration distribution and residual stress distribution results.

[0050] According to Figure 3 , Figure 4 , it can be known that the finite element analysis method of low-temperature thermo-chemical carburized / nitrogen layer diffusion and stress distribution established by the present application can accurately calculate the distribution of carbon / nitrogen content in the carburized layer, the distribution results are consistent with the test results, and the residual stress distribution results are consistent with the test measured values, which can realize the accurate prediction of the mechanical behavior of the carburized / nitrogen layer material, provide theoretical basis and technical support for the performance optimization and process improvement of complex geometric components.

[0051] The above merely describes the preferred embodiments of the present application, and it should be pointed out that those skilled in the art can make several improvements and refinements without departing from the principles of the present application, and these improvements and refinements should also be considered as falling within the protection scope of the present application.

Claims

1. A finite element analysis method for diffusion and stress distribution in a low-temperature thermochemical carburizing / nitriding layer, characterized in that, Includes the following steps: S1. Construct a 3D model and perform mesh generation and refinement. The specific method is as follows: In the 3D model, a mesh refinement strategy is adopted in the depth direction of the carburized / nitrided layer. High-density meshes are divided in the carburized / nitrided layer region to ensure calculation accuracy. Larger meshes are used in the matrix region and in parts far from the carburized layer to reduce the overall calculation workload and improve modeling efficiency. S2. Construct a calculation model for mass and heat transfer parameters related to carbon / nitrogen concentration: A computational model is constructed based on the unsteady diffusion coefficients of carbon / nitrogen defined by the UMATHT module to simulate the influence of concentration gradient on diffusion behavior. The diffusion coefficients are dynamically correlated with the concentration field. S3. Obtain concentration-related material parameters, including: Elastic modulus, yield strength, and hardening index of the carburized layer material under different carburizing / nitrogenating concentrations; Expansion coefficients under different carburizing / nitrogenating concentrations; S4. Input the material parameters from S3 into the finite element model to obtain the concentration-expansion coefficient relationship model; construct a dynamic correlation database between the concentration field and material parameters for defining material properties in finite element analysis; S5. The carbon concentration distribution and residual stress distribution in the three-dimensional component are obtained through numerical simulation; the simulation results are compared with the concentration distribution and residual stress test data obtained from experiments to verify the accuracy of the concentration distribution and residual stress distribution results. The minimum size of the high-density average grid in the carburized / nitrided layer region of S1 is 0.001~0.005 mm, and the maximum grid size does not exceed 1 / 10 of the total depth of the carburized layer. The grid size is divided into increments from the carburized layer region to the matrix region. Specifically, the minimum grid size near the carburized layer region is 0.001~0.005 mm, and the grid size near the matrix region is 0.1~0.5 mm.

2. The finite element analysis method according to claim 1, characterized in that, The three-dimensional model was created in ABAQUS finite element software.

3. The finite element analysis method according to claim 1, characterized in that, The calculation model in S2 is as follows: In the formula, D represents the diffusion coefficient. Let exp() represent the diffusion constant, exp() represent the exponential term, k represent the proportionality constant, and C represent the current concentration. This indicates the maximum concentration.

4. The finite element analysis method according to claim 1, characterized in that, The elastic modulus, yield strength, and hardening index mentioned in S3 are obtained through nanoindentation experiments; The coefficient of thermal expansion was obtained using an X-ray diffractometer.

5. The finite element analysis method according to claim 1, characterized in that, In step S5, an electron probe microanalyzer is used to obtain the concentration distribution in the carburized / nitrided layer, and X-ray diffraction is used to measure the residual stress in the carburized / nitrided layer. The obtained experimental data are then compared and analyzed with the numerical simulation results.

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