A method for obtaining nonlinear mixed hardening model of welding materials

Abstract

The present invention provides a method for obtaining a nonlinear mixed hardening model of a welding material, comprising the following steps: performing an isothermal multi-stage cyclic tension and compression test on the welding material to obtain a stress-strain curve of the welding material at a set temperature; calculating the yield strength of the welding material in the tensile stage, the maximum tensile stress, and the yield strength in the compression stage; calculating the internal stress, the kinematic hardening model ratio, and the isotropic hardening model ratio; determining the model parameters of the A-F nonlinear kinematic hardening model; converting P Kin X+P Iso Y is used as the nonlinear mixed hardening model of the welding material. Based on the hardening model criteria and a large number of experimental verifications, this method derives the ratio calculation method of the kinematic hardening model and the isotropic hardening model, which can quantitatively determine the ratio of the isotropic hardening model; combined with the A-F nonlinear kinematic hardening model and the isotropic hardening model, the established nonlinear mixed hardening model can accurately calculate the thermoelastic-plastic stress-strain evolution process of the material during welding.

Description

Technical Field

[0001] The invention relates to the technical field of welding mechanics, and in particular to a method for obtaining a nonlinear mixed hardening model of welding materials. Background Art

[0002] Advanced ultra-supercritical thermal power generation technology is a coal-fired power generation technology with low carbon emissions, high power generation efficiency and low product pollution. Due to the harsh working temperature and pressure conditions, higher requirements are placed on the high-temperature mechanical properties, high-temperature corrosion resistance and high-temperature creep properties of materials. Welding residual stress and deformation will affect the service performance of key components of advanced ultra-supercritical units (such as main pipelines and high-temperature rotors). Comprehensive evaluation of welding residual stress of key components has important research value for key component welding structure design, process optimization and service performance evaluation.

[0003] The use of a combination of scientific welding numerical simulation technology and a small amount of experimental verification to transform the research model from "theory-experiment-production" to "theory-computer simulation-production" is the development trend of today's information age. It provides important support for new product design, process formulation and optimization, structural safety assessment and failure analysis, etc., which not only saves a lot of manpower and material resources required for experiments, but also improves the scientific level of welding thermal processing. However, in welding numerical simulation, the traditional isotropic hardening model and kinematic hardening model cannot accurately predict the thermoelastic-plastic stress-strain relationship of the material in the welding process, resulting in a large deviation between the predicted welding residual stress and the actual value. The recently developed nonlinear mixed hardening model can more accurately characterize the thermoelastic-plastic stress-strain relationship of the welding process. Although it has been successfully applied to the welding numerical simulation of key nuclear power components (such as the welding numerical simulation of the safety end), when establishing the nonlinear mixed hardening model, the proportion of the isotropic hardening model is determined by the assumption method, lacking scientific and reasonable experimental means and calculation methods.

[0004] Therefore, how to establish a stress-strain test characterization method for welding process materials, propose a scientific and reasonable isotropic hardening model proportional calculation method, and construct an accurate nonlinear mixed hardening model to characterize the thermal elastic-plastic stress-strain relationship of welding process materials has become an important issue that needs to be urgently solved in the industry. Summary of the invention

[0005] The present invention provides a method for obtaining a nonlinear mixed hardening model of a welding material, so as to solve the technical problem that the nonlinear mixed hardening model of a welding material cannot accurately characterize the thermal elastic-plastic stress-strain relationship of the welding material.

[0006] In order to solve the above technical problems, the present invention provides a method for obtaining a nonlinear mixed hardening model of welding materials, the method comprising the following steps:

[0007] S1. Perform an isothermal multi-stage cyclic tension and compression test on the welding material to obtain a stress-strain curve of the welding material at a set temperature;

[0008] S2. Extract the stress-strain curve of each cycle and calculate the yield strength σ of the tensile stage of each cycle yn , maximum tensile stress σ Fmaxn and the yield strength σ in the compression stage Rn , the yield strength σ of the tensile stage of all cycles yn The average value of the yield strength σ of the welding material in the tensile stage y , the maximum tensile stress σ of all cycles Fmaxn The average value is taken as the maximum tensile stress σ of the welding material Fmax , the yield strength σ of the compression stage of all cycles Rn The average value of the yield strength σ of the welding material in the compression stage R , wherein the number of stages of the isothermal multi-stage cyclic tension and compression test is ≥ 2, and the number of cycles of each stage n is ≥ 2;

[0009] S3, according to the formula Calculate the internal stress <σ> according to the formula Calculate the kinematic hardening model proportion P Kin According to the formula P Iso =1-P Kin Calculate the isotropic hardening model proportion P Iso ;

[0010] S4, fitting the AF nonlinear kinematic hardening model using the data of the first cycle stress-strain curve to determine the model parameters of the AF nonlinear kinematic hardening model;

[0011] S5. Kin X+P Iso Y is used as the nonlinear mixed hardening model of the welding material, wherein X represents the AF nonlinear kinematic hardening model after the parameters are determined, and Y represents the isotropic hardening model.

[0012] Optionally, after step S5, the following steps are also included:

[0013] S6. Based on the elastic-plastic finite element method, the isothermal multi-stage cyclic tension and compression test is simulated, and the simulated and measured stress-strain curves are compared to verify the accuracy of the nonlinear mixed hardening model of the welding material.

[0014] Optionally, after step S6, the following steps are also included:

[0015] S7. Repeat steps S1 to S6 to obtain a nonlinear mixed hardening model of the welding material at different set temperatures.

[0016] Optionally, the welding material is in the shape of a round rod, and both ends of the welding material are clamping ends, and the clamping ends are provided with threads.

[0017] Optionally, the heating rate of the isothermal multi-stage cyclic tension and compression test is 5 to 15°C / s, and the welding material is wrapped with thermal insulation cotton to ensure the uniform temperature of parallel sections of the welding material.

[0018] Optionally, three temperature sensors for collecting temperature are evenly arranged on the parallel sections of the welding material.

[0019] Optionally, the multi-stage load of the isothermal multi-stage cyclic tension and compression test is not less than 2 levels, the number of cyclic tension and compression under each level of load is not less than 2 times, and the strain rate is not higher than 5×10 -4 / s; The first level strain should be greater than the elastic strain of the welding material at the set temperature to ensure that the welding material undergoes plastic deformation during the test, and the subsequent strains increase step by step. The set maximum strain should ensure that the welding material does not undergo bending deformation during the compression process.

[0020] The present invention provides a method for obtaining a nonlinear mixed hardening model of welding materials. Based on the hardening model criteria and a large number of experimental verifications, a method for calculating the proportion of a kinematic hardening model and an isotropic hardening model is derived, which can quantitatively determine the proportion of the isotropic hardening model and make up for the shortcomings of the existing assumption method. Combining the AF nonlinear kinematic hardening model and the isotropic hardening model, the established nonlinear mixed hardening model can accurately calculate the thermal elastic-plastic stress-strain evolution process of the material during the welding process, and improve the accuracy of the welding numerical simulation results. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a flow chart of a method for obtaining a nonlinear mixed hardening model of welding materials provided by an embodiment of the invention.

[0022] Figure 2 It is a schematic diagram of parameters of an isothermal multi-stage cyclic tension and compression test of a nickel-iron-based high-temperature alloy provided in one embodiment of the present invention.

[0023] Figure 3 This is a cyclic stress-strain curve of a nickel-iron-based high-temperature alloy at 400° C. provided in one embodiment of the present invention.

[0024] Figure 4 4 is a graph showing the relationship between the maximum tensile stress and the number of cycles of a nickel-iron-based high-temperature alloy at 400° C. provided in one embodiment of the present invention.

[0025] Figures 5a to 5c This is a comparison diagram of stress-strain curves calculated by different hardening models and measured values ​​of a nickel-iron-based high-temperature alloy at 400° C. provided by an embodiment of the present invention: Figure 5ais the comparison diagram corresponding to the isotropic hardening model; Figure 5b Comparison diagram corresponding to the nonlinear kinematic hardening model; Figure 5c It is a comparison diagram corresponding to the nonlinear mixed hardening model provided by the present invention.

[0026] Figure 6 This is a relationship diagram between the ratio of the kinematic hardening model of a nickel-iron-based high-temperature alloy and temperature provided by an embodiment of the present invention.

[0027] Figures 7a to 7h It is a comparison diagram of the predicted cyclic stress-strain curves of the nickel-iron-based high-temperature alloy provided by one embodiment of the present invention at room temperature, 200°C, 300°C, 400°C, 500°C, 600°C, 700°C and 800°C and the measured values. DETAILED DESCRIPTION

[0028] In order to make the purpose, advantages and features of the present invention more clear, a method for obtaining a nonlinear mixed hardening model of welding materials proposed by the present invention is further described in detail below in conjunction with the accompanying drawings. It should be noted that the accompanying drawings are all in a very simplified form and are not in precise proportions, and are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0029] like Figure 1 As shown, this embodiment provides a method for obtaining a nonlinear mixed hardening model of welding materials, and the method includes the following steps:

[0030] S1. Perform an isothermal multi-stage cyclic tension and compression test on the welding material to obtain a stress-strain curve of the welding material at a set temperature.

[0031] Nickel-iron-based alloys in high-temperature alloys have good high-temperature mechanical properties, high-temperature corrosion resistance and good creep properties, and are expected to play an important role in ultra-supercritical thermal power units, but the research on the constitutive relationship of welding mechanics is still in a blank stage. This embodiment mainly tests and verifies a certain grade of nickel-iron-based high-temperature alloy.

[0032] Optionally, the nickel-iron-based high-temperature alloy specimen is processed into a round bar specimen with a parallel section length of 16 mm, a diameter of 8 mm, a clamping end length of 27 mm, a thread specification of M16×1, and a transition arc radius of 20 mm between the clamping end and the parallel section. This makes it easier to fix the specimen and meet the needs of fixing the temperature sensor and extensometer.

[0033] Install the sample on the high temperature fatigue testing machine, and use an infrared heating furnace to heat the sample to the set temperature, with a heating rate of 5 to 15°C / s. The set temperature can be selected from room temperature to the allowable temperature range according to the requirements of the high temperature fatigue testing machine. Three contact thermocouple temperature sensors can be evenly arranged at the upper, middle and lower positions of the parallel section of the sample, and the sample can be wrapped with insulation cotton. The heating rate is set to 15°C / s and the insulation is maintained for 20 minutes to ensure that the temperature difference within the 16mm range of the parallel section does not exceed ±5°C.

[0034] The extensometer was fixed to the parallel section of the specimen and strain control was adopted with the strain rate not higher than 5×10 -4 / s, which can be 2×10 -4 / s, and conduct isothermal multi-stage load cyclic tension and compression tests. Figure 2 It is a schematic diagram of an isothermal multi-stage load cyclic tension and compression test at 400°C provided in this embodiment. The multi-stage load is designed with 3 levels of load, and the strain ranges are ±0.5%, ±1.0% and ±1.5%, respectively, and the number of cycles for each level is 10 times.

[0035] During the test, temperature, stress and strain data are collected and stress-strain curves are drawn. Figure 3 It is the cyclic stress-strain curve at 400℃.

[0036] The relationship between the maximum tensile stress and the number of cycles can be obtained according to the stress-strain curve to determine the cyclic hardening characteristics of the welding material. Figure 3 Extract the maximum tensile stress σ under each cycle Fmaxn , plot σ Fmaxn The relationship with the number of cycles is as follows: Figure 4 As shown, the number of stages of the isothermal multi-stage cyclic tension and compression test is equal to 3, and the number of cycles in each stage is n=10. It can be seen from the figure that at 400°C, with the increase of the number of cycles and the strain, the maximum tensile stress gradually increases, indicating that the nickel-iron-based high-temperature alloy has obvious strain hardening characteristics at 400°C.

[0037] S2. Extract the stress-strain curve of each cycle and calculate the yield strength σ of the tensile stage of each cycle yn , maximum tensile stress σ Fmaxn and the yield strength σ in the compression stage Rn , the yield strength σ of the tensile stage of all cycles yn The average value of the yield strength σ of the welding material in the tensile stage y , the maximum tensile stress σ of all cycles Fmaxn The average value is taken as the maximum tensile stress σ of the welding material Fmax , the yield strength σ of the compression stage of all cycles RnThe average value of the yield strength σ of the welding material in the compression stage R , wherein the number of stages of the isothermal multi-stage cyclic tension and compression test is ≥2, and the number of cycles of each stage n is ≥2.

[0038] σ yn , σ Fmaxn , σ Rn The calculation process of belongs to the prior art and will not be described in detail here. The maximum tensile stress σ Fmaxn is the tensile stress corresponding to the positive tensile strain reaching the set strain value under each cyclic load, Where, m = the number of stages of isothermal multi-stage cyclic tension and compression test*n, Figure 4 m = 3*10 = 30 times. y and σ R The calculation process of taking the average value is similar to σ Fmax The calculation process for taking the average value is similar. The choice of the reverse strain offset determines the yield strength σ in the compression stage R The value of internal stress <σ> and the kinematic hardening model ratio P Kin The reverse strain offset has an important influence. The reverse strain offset value range may be 0.02% to 0.2%, and specifically, a 0.1% reverse strain offset may be selected.

[0039] S3, according to the formula Calculate the internal stress <σ> according to the formula Calculate the kinematic hardening model proportion P Kin According to the formula P Iso =1-P Kin Calculate the isotropic hardening model proportion P Iso .

[0040] The formula derivation process of this step is as follows:

[0041] According to the isotropic hardening model criterion, the yield stress σ in the reverse compression stage RI It can be expressed as:

[0042] |σ RI |=σ Fmax (1)

[0043] According to the kinematic hardening model criterion, the yield stress σ in the reverse compression stage RK Cocoa can be expressed as:

[0044] |σ RK |=2σ y -σ Fmax (2)

[0045] Assume that the kinematic hardening model ratio is P Kin , then the yield strength σ in the compression stage RIt can be expressed as:

[0046] |σ R |=(1-P Kin )|σ RI |+P Kin |σ RK | (3)

[0047] Substituting equation (1) and equation (2) into equation (3), we get:

[0048]

[0049] Let <σ> be the internal stress, <σ> can be expressed as:

[0050]

[0051] Thus we get:

[0052]

[0053] P Iso =1-P Kin (7)

[0054] S4. Using the data of the first cycle stress-strain curve to fit the AF nonlinear kinematic hardening model, the model parameters of the AF nonlinear kinematic hardening model are determined.

[0055] The AF nonlinear kinematic hardening model can be expressed as:

[0056]

[0057] in, is the plastic stress change rate, is the equivalent plastic strain, is the plastic strain change rate, is the plastic stress, is the equivalent plastic strain change rate, h 1 、h 2 and k 2 is the model parameter, h 1 、h 2 and k 2 It can be obtained by fitting the measured stress-strain data of the first cycle.

[0058] S5. Kin X+P Iso Y is used as the nonlinear mixed hardening model of the welding material, wherein X represents the AF nonlinear kinematic hardening model after the parameters are determined, and Y represents the isotropic hardening model.

[0059] The present embodiment provides a method for obtaining a nonlinear mixed hardening model of welding materials. Based on the hardening model criteria and a large number of experimental verifications, a method for calculating the ratio of the kinematic hardening model and the isotropic hardening model is derived, which can quantitatively determine the ratio of the isotropic hardening model and make up for the shortcomings of the existing assumption method. The nonlinear mixed hardening model established by combining the AF nonlinear kinematic hardening model and the isotropic hardening model can accurately calculate the thermal elastic-plastic stress-strain evolution process of the material during welding, thereby improving the accuracy of the welding numerical simulation results.

[0060] Optionally, step S5 further includes the following steps: S6, simulating an isothermal multi-stage cyclic tension-compression test based on an elastic-plastic finite element method, comparing simulated and measured stress-strain curves, and verifying the accuracy of the nonlinear mixed hardening model of the welding material.

[0061] Finite element software can be used to simulate the isothermal multi-stage cyclic tension and compression test of nickel-iron-based high-temperature alloy at 400°C, and the isotropic hardening model, AF nonlinear kinematic hardening model, nonlinear mixed hardening model and measured cyclic stress-strain curves can be compared. Figures 5a to 5c As shown, the isotropic hardening model obviously overestimates the stress evolution, the AF nonlinear kinematic hardening model underestimates the stress evolution, and the nonlinear mixed hardening model is most consistent with the measured cyclic stress-strain curve. This can verify the reliability of the method proposed in the present invention for obtaining a nonlinear mixed hardening model of welding materials.

[0062] Optionally, after step S6, the following steps are also included: S7, repeating steps S1 to S6 to obtain a nonlinear mixed hardening model of the welding material at different set temperatures.

[0063] In order to further verify the accuracy of the method proposed in the present invention for obtaining the nonlinear mixed hardening model of welding materials, Figure 7a to Figure 7h As shown, the present invention completed isothermal multi-stage cyclic tension and compression tests at room temperature, 200°C, 300°C, 400°C, 500°C, 600°C, 700°C and 800°C, and calculated the proportions of the kinematic hardening model and the isotropic hardening model at different temperatures. Figure 6 is the relationship between the proportion of the kinematic hardening model and temperature. As can be seen from the figure, with the increase of temperature, the proportion of the kinematic hardening model shows a linear and slow increasing trend. At the same time, according to the cyclic stress-strain curve, the parameters of the AF kinematic hardening model at different temperatures are fitted, combined with the determination of the isotropic hardening model proportion P Iso A nonlinear mixed hardening model was established based on the isotropic hardening model. By comparing the predicted cyclic stress-strain curves at different temperatures with the measured values, the stress-strain curves calculated by the nonlinear mixed hardening model were consistent with the measured values.

[0064] In summary, the present invention provides a method for obtaining a nonlinear mixed hardening model of welding materials. Based on the hardening model criteria and a large number of experimental verifications, a method for calculating the proportion of the kinematic hardening model and the isotropic hardening model is derived, which can quantitatively determine the proportion of the isotropic hardening model and make up for the shortcomings of the existing assumption method. Combining the AF nonlinear kinematic hardening model and the isotropic hardening model, the established nonlinear mixed hardening model can accurately calculate the thermal elastic-plastic stress-strain evolution process of the material during the welding process, and improve the accuracy of the welding numerical simulation results.

[0065] The above description is only a description of the preferred embodiments of the present invention, and is not intended to limit the scope of the present invention. Any changes or modifications made by a person skilled in the art based on the above disclosure shall fall within the protection scope of the present invention.

Claims

1. A method for obtaining a nonlinear mixed hardening model of welding materials, characterized in that: The method comprises the following steps: S1. Perform an isothermal multi-stage cyclic tension and compression test on the welding material to obtain a stress-strain curve of the welding material at a set temperature; S2. Extract the stress-strain curve of each cycle and calculate the yield strength σ of the tensile stage of each cycle yn , maximum tensile stress σ Fmaxn and the yield strength σ in the compression stage Rn , the yield strength σ of the tensile stage of all cycles yn The average value of the yield strength σ of the welding material in the tensile stage y , the maximum tensile stress σ of all cycles Fmaxn The average value is taken as the maximum tensile stress σ of the welding material Fmax , the yield strength σ of the compression stage of all cycles Rn The average value of the yield strength σ of the welding material in the compression stage R , wherein the number of stages of the isothermal multi-stage cyclic tension and compression test is ≥ 2, and the number of cycles of each stage n is ≥ 2; S3, according to the formula Calculate the internal stress <σ> according to the formula Calculate the kinematic hardening model proportion P Kin According to the formula P Iso =1-P Kin Calculate the isotropic hardening model proportion P Iso ; S4. Fitting the AF nonlinear kinematic hardening model with the data of the first cycle stress-strain curve to determine the model parameters of the AF nonlinear kinematic hardening model; the AF nonlinear kinematic hardening model is expressed as: in, is the plastic stress change rate, is the equivalent plastic strain, is the plastic strain change rate, is the plastic stress, is the equivalent plastic strain change rate, h 1 、h 2 and k 2 is the model parameter, h 1 、h 2 and k 2 It is obtained by fitting the measured stress-strain data of the first cycle; S5. Kin X+P Iso Y is used as the nonlinear mixed hardening model of the welding material, wherein X represents the AF nonlinear kinematic hardening model after the parameters are determined, and Y represents the isotropic hardening model.

2. A method for obtaining a nonlinear mixed hardening model of welding materials according to claim 1, characterized in that: After step S5, the following steps are also included: S6. Based on the elastic-plastic finite element method, the isothermal multi-stage cyclic tension and compression test is simulated, and the simulated and measured stress-strain curves are compared to verify the accuracy of the nonlinear mixed hardening model of the welding material.

3. A method for obtaining a nonlinear mixed hardening model of welding materials according to claim 2, characterized in that: After step S6, the following steps are also included: S7. Repeat steps S1 to S6 to obtain a nonlinear mixed hardening model of the welding material at different set temperatures.

4. The method for obtaining a nonlinear mixed hardening model of welding materials according to claim 1, characterized in that: The welding material is in the shape of a round rod, and both ends of the welding material are clamping ends, and the clamping ends are provided with threads.

5. The method for obtaining a nonlinear mixed hardening model of welding materials according to claim 1, characterized in that: The heating rate of the isothermal multi-stage cyclic tension and compression test is 5-15°C / s, and the welding material is wrapped with thermal insulation cotton to ensure the uniform temperature of the parallel sections of the welding material.

6. A method for obtaining a nonlinear mixed hardening model of welding materials according to claim 5, characterized in that: Three temperature sensors for collecting temperature are evenly arranged on the parallel sections of the welding material.

7. A method for obtaining a nonlinear mixed hardening model of welding materials according to claim 1, characterized in that: The multi-level load of the isothermal multi-level cyclic tension and compression test shall not be less than 2 levels, the number of cyclic tension and compression under each level of load shall not be less than 2 times, and the strain rate shall not be higher than 5×10 -4 / s; The first level strain should be greater than the elastic strain of the welding material at the set temperature to ensure that the welding material undergoes plastic deformation during the test, and the subsequent strains increase step by step. The set maximum strain should ensure that the welding material does not undergo bending deformation during the compression process.

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