Finite element analysis method for low-temperature thermochemical carburization / nitrogen layer diffusion and stress distribution

Abstract

The invention provides a finite element analysis method for low-temperature thermochemical carburization / nitrogen layer diffusion and stress distribution, 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 carrying out grid division and refinement; s2, building a mass transfer and heat transfer parameter definition calculation model related to the carbon / nitrogen concentration; S3, obtaining material parameters related to the concentration, wherein the material parameters comprise the elastic modulus, the yield strength and the hardening index of the infiltrated layer material under different carburizing / nitrogen concentration conditions; expansion coefficients under different carburization / nitrogen concentration conditions; s4, inputting the material parameters in the step S3 into a finite element model; s5, carbon concentration distribution and residual stress distribution in the three-dimensional component are obtained through numerical simulation, distribution of the carbon / nitrogen content in the permeation layer can be accurately calculated, the distribution result is consistent with a test result, and the residual stress distribution result is consistent with a test measurement value.

Description

Technical Field

[0001] The invention belongs to the technical field of material mechanical behavior analysis and finite element simulation, and specifically relates to a finite element analysis method for low-temperature thermochemical carburizing / nitriding layer diffusion and stress distribution. Background Art

[0002] Low-temperature thermochemical surface strengthening technology is a surface modification technology that infiltrates carbon, nitrogen or carbonitriding elements into the metal surface at a temperature lower than the traditional phase change temperature. This technology forms a carburized / nitrided layer with a thickness of usually no more than 50 microns on the metal surface through a diffusion process. This technology can maintain the toughness of the matrix while significantly improving the hardness and wear resistance of the surface. The high hardness of the carburized layer comes from the combination reaction of the alloying elements and the infiltrated elements, while the high compressive residual stress is due to the volume expansion and stress field reconstruction caused by the diffusion of the carburized layer. The high compressive residual stress of the carburized layer can significantly improve the surface fatigue performance of the material, making this technology widely used in precision machinery parts, medical devices, aerospace components and transmission gears in the automotive industry.

[0003] However, the current modeling methods for the behavior of the carburized layer during low-temperature thermochemical surface strengthening are mainly limited to the two-dimensional scale. Peng et al. established a carbon diffusion method for low-temperature thermochemical carburized layers 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 carburized layer, resulting in low prediction accuracy of the residual stress in the carburized layer. Since the high compressive residual stress of the carburized layer is crucial to the improvement of fatigue performance, accurate simulation of the distribution of residual stress is of great significance for optimizing the process and improving performance. Traditional two-dimensional modeling cannot fully describe the three-dimensional stress distribution characteristics of complex components, which limits its application in complex geometric parts.

[0004] Therefore, a three-dimensional modeling method is needed to simulate and predict the residual stress distribution characteristics in the carburized layer to achieve accurate prediction of the mechanical behavior of the carburized / nitrided layer material, and provide a theoretical basis and technical support for performance optimization and process improvement of complex geometric components. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a finite element analysis method for low-temperature thermochemical carburizing / nitriding layer diffusion and stress distribution. The finite element analysis method for low-temperature thermochemical carburizing / nitriding layer diffusion and stress distribution established by the present invention can accurately calculate the distribution of carbon / nitrogen content in the carburized layer, and its distribution results are consistent with the experimental results and the residual stress distribution results are consistent with the experimental measurement values.

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

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

[0008] S1. Build a 3D model and perform mesh division and refinement. The specific method is as follows:

[0009] In the 3D model, a grid refinement strategy in the depth direction of the carburized / nitrided layer is adopted to divide the carburized / nitrided layer area into high-density grids to ensure the calculation accuracy; in the matrix area and the part far away from the carburized layer, a larger grid is used to reduce the overall calculation amount and improve the modeling efficiency;

[0010] S2. Construct a computational model to define mass transfer and heat transfer parameters related to carbon / nitrogen concentration:

[0011] Based on the UMATHT module, a computational model is constructed to define the non-steady-state diffusion coefficients of carbon and nitrogen to simulate the effect of concentration gradient on diffusion behavior. The diffusion coefficient is dynamically related to the concentration field.

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

[0013] Elastic modulus, yield strength and hardening index of carburized layer materials under different carburizing / nitrogen concentration conditions;

[0014] Expansion coefficient under different carburizing / nitrogen concentration conditions;

[0015] S4. Input the material parameters in S3 into the finite element model to obtain a concentration-expansion coefficient relationship model; construct a dynamic correlation database between concentration field and material parameters for material performance definition in 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 from the experiment to verify the accuracy of the concentration distribution and residual stress distribution results.

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

[0018] Preferably, the minimum size of the high-density average grid for dividing the carburized / nitrided layer area in 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; an incremental grid size division method is set from the carburized layer area to the matrix area, specifically, the minimum grid size close to the carburized layer area is 0.001-0.005 mm, and the grid size close to the matrix area is 0.1-0.5 mm.

[0019] Preferably, the calculation model in S2 is:

[0020]

[0021] In the formula, D represents the diffusion coefficient, D0 represents the diffusion constant, exp() represents the exponential term, k represents the proportionality coefficient, C represents the current concentration, and C max Indicates the maximum concentration.

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

[0023] The expansion coefficients at different concentrations were obtained by X-ray diffractometer.

[0024] Preferably, in S5, an electron probe microanalyzer is used to obtain the concentration distribution in the carburized / nitrided layer, and an X-ray diffraction technique is used to measure the residual stress in the carburized / nitrided layer; and the obtained experimental data is compared and analyzed with the numerical simulation results.

[0025] Contains at least the following beneficial technical effects:

[0026] (1) The low-temperature thermochemical carburizing / nitriding layer diffusion and stress distribution finite element analysis method established by the present invention can accurately calculate the distribution of carbon / nitrogen content in the carburized layer, and the distribution results are consistent with the test results.

[0027] (2) The low-temperature thermochemical carburizing / nitriding layer finite element model established by the present invention can obtain the compressive residual stress distribution characteristics in a very thin carburized layer in a large-scale finite element model, and the residual stress distribution results are consistent with the experimental measurement values. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The flow chart of the finite element analysis method for low temperature thermochemical carburizing / nitriding layer diffusion and stress distribution;

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

[0030] Figure 3 The carbon concentration distribution result in the carburized layer obtained in step S2 of Example 1 is compared with the carbon concentration result obtained by the experimental value;

[0031] Figure 4 The residual stress distribution result in the carburized layer obtained in step S5 of Example 1 is compared with the residual stress result obtained in the experiment. DETAILED DESCRIPTION

[0032] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0033] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0034] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0035] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present application description and examples are exemplary only.

[0036] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0037] The "room temperature" and "normal temperature" mentioned in the present invention are all calculated as 25±2°C unless otherwise specified.

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

[0039] The embodiment of the present invention uses austenitic stainless steel 316L, a cubic piece with a specification of 10mm×10mm×10mm, and the low-temperature thermochemical surface treatment process used is low-temperature gas carburizing.

[0040] Example 1

[0041] This embodiment provides a low temperature thermochemical carburizing / nitriding layer diffusion and stress distribution finite element analysis method, comprising the following steps:

[0042] S1. A 10mm×10mm×10mm three-dimensional model was established in the ABAQUS finite element software. The mesh size of the carburized layer was averagely distributed, and the mesh size from the carburized layer to the matrix was gradient-increasing. Specifically, the mesh size in the depth direction of the carburized layer was 0.002; the mesh size of the carburized layer to the matrix was gradient-increasing. Specifically, the mesh size of the carburized layer was 0.002mm, and the mesh size of the matrix was 0.5mm. The mesh division results are shown in Figure 2 .

[0043] S2. Based on step S1, the non-steady-state diffusion coefficient of carbon diffusion is defined based on the UMATHT module in ABAQUS software to simulate the influence of carbon concentration gradient on diffusion behavior. The definition of the non-steady-state diffusion coefficient adopts 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 proportionality coefficient, C represents the current carbon concentration, and C max Indicates the maximum carbon concentration.

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

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

[0048] S4. Input the material parameters in S3 into the finite element model to obtain a concentration-expansion coefficient relationship model; construct a dynamic correlation database between concentration field and material parameters for material performance definition in finite element analysis;

[0049] S5. Use the established low-temperature thermochemical carburized layer finite element model to predict the carbon concentration distribution and residual stress distribution in the carburized layer; compare the simulation results with the carbon concentration distribution obtained from the carburizing experiment (see Figure 3 ) and residual stress (see Figure 4 ) test data 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 low-temperature thermochemical carburizing / nitriding layer diffusion and stress distribution finite element analysis method established by the present invention can accurately calculate the distribution of carbon / nitrogen content in the carburized layer, and its distribution results are consistent with the experimental results, and the residual stress distribution results are consistent with the experimental measurement values. It can achieve accurate prediction of the mechanical behavior of the carburizing / nitriding layer material, and provide a theoretical basis and technical support for performance optimization and process improvement of complex geometric components.

[0051] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A finite element analysis method for low temperature thermochemical carburizing / nitriding layer diffusion and stress distribution, characterized in that: The following steps are involved: S1. Build a 3D model and perform mesh division and refinement. The specific method is as follows: In the 3D model, a grid refinement strategy in the depth direction of the carburized / nitrided layer is adopted to divide the carburized / nitrided layer area into high-density grids to ensure the calculation accuracy; in the matrix area and the part far away from the carburized layer, a larger grid is used to reduce the overall calculation amount and improve the modeling efficiency; S2. Construct a computational model to define mass transfer and heat transfer parameters related to carbon / nitrogen concentration: Based on the UMATHT module, a computational model is constructed to define the non-steady-state diffusion coefficients of carbon and nitrogen to simulate the effect of concentration gradient on diffusion behavior. The diffusion coefficient is dynamically related to the concentration field. S3. Obtaining material parameters related to concentration, the material parameters including: Elastic modulus, yield strength and hardening index of carburized layer materials under different carburizing / nitrogen concentration conditions; Expansion coefficient under different carburizing / nitrogen concentration conditions; S4. Input the material parameters in S3 into the finite element model to obtain a concentration-expansion coefficient relationship model; construct a dynamic correlation database between concentration field and material parameters for material performance definition in finite element analysis; 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 from the experiment to verify the accuracy of the concentration distribution and residual stress distribution results.

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

3. The finite element analysis method according to claim 1, characterized in that: The minimum size of the high-density average grid for dividing the carburized / nitrided layer area in 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; an incremental grid size division method is set from the carburized layer area to the matrix area, specifically, the minimum grid size close to the carburized layer area is 0.001-0.005 mm, and the grid size close to the matrix area is 0.1-0.5 mm.

4. The finite element analysis method according to claim 1, characterized in that: The calculation model in S2 is: In the formula, D represents the diffusion coefficient, D0 represents the diffusion constant, exp() represents the exponential term, k represents the proportionality coefficient, C represents the current concentration, and C max Indicates the maximum concentration.

5. The finite element analysis method according to claim 1, characterized in that: The elastic modulus, yield strength and hardening index in S3 are obtained by nanoindentation experiment; The expansion coefficients mentioned are obtained by X-ray diffractometry.

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

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