CN114121189B - A method for obtaining nonlinear mixed hardening model of welding materials - Google Patents
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Abstract
本发明提供的一种获取焊接材料非线性混合硬化模型的方法，包括以下步骤：对焊接材料进行等温多级循环拉压试验，获取焊接材料在设定温度下的应力应变曲线；计算焊接材料的拉伸阶段的屈服强度、最大拉应力和压缩阶段的屈服强度；计算内应力、随动硬化模型比例、等向硬化模型比例；确定A‑F非线性随动硬化模型的模型参数；将PKinX+PIsoY作为焊接材料的非线性混合硬化模型。本方法基于硬化模型准则和大量的试验验证，推导随动硬化模型和等向硬化模型比例计算方法，可以定量地确定等向硬化模型比例；结合A‑F非线性随动硬化模型和等向硬化模型，建立的非线性混合硬化模型可准确地计算焊接过程中材料的热弹塑性应力应变演化过程。
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; and using P Kin X+P Iso Y as the nonlinear mixed hardening model of the welding material. Based on the hardening model criterion and a large number of experimental verifications, this method derives a method for calculating the ratio of the kinematic hardening model and the isotropic hardening model, and can quantitatively determine the ratio of the isotropic hardening model; combining the A-F nonlinear kinematic hardening model and the isotropic hardening model, the nonlinear mixed hardening model established can accurately calculate the thermoelastic-plastic stress-strain evolution process of the material during welding.
Description
Technical Field
The invention relates to the technical field of welding mechanics, in particular to a method for obtaining a nonlinear hybrid hardening model of a welding material.
Background
The advanced ultra-supercritical thermal power generation technology is a coal-fired power generation technology with low carbon emission, high power generation efficiency and small product pollution. Because of the harsh working temperature and pressure conditions, the high-temperature mechanical property, high-temperature corrosion resistance and high-temperature creep property of the material are required to be improved. The welding residual stress and deformation influence the service performance of key components (such as a main pipeline and a high-temperature rotor) of the advanced ultra-supercritical unit, and the comprehensive evaluation of the welding residual stress of the key components has important research values for the design of the welding structure of the key components, the process optimization and the service performance evaluation.
The method of combining scientific welding numerical simulation technology and a small amount of test verification is adopted, so that a research mode is changed from theory-test-production into theory-computer simulation-production, the method is a trend of development in the current information age, important support is provided for new product design, process formulation and optimization, structural safety assessment, failure analysis and other aspects, a large amount of manpower and material resources required by tests are saved, and the scientific level of welding hot working is improved. However, in the welding numerical simulation, the traditional isotropic hardening model and the follow-up hardening model cannot accurately predict the thermoelastic plastic stress-strain relation of the materials in the welding process, so that the predicted welding residual stress has larger deviation from the actual value. The recently developed nonlinear hybrid hardening model can more accurately represent the thermoelastic-plastic stress-strain relation of the welding process, and although the nonlinear hybrid hardening model is successfully applied to welding numerical simulation of nuclear power key parts (such as welding numerical simulation of a safety end), the isotropic hardening model proportion is determined by adopting a false method when the nonlinear hybrid hardening model is established, and a scientific and reasonable test means and a calculation method are lacked.
Therefore, how to establish a stress-strain test characterization method of a welding process material, and to provide a scientific and reasonable equidirectional hardening model proportion calculation method, to construct an accurate nonlinear hybrid hardening model to characterize the thermal elastoplastic stress-strain relationship of the welding process material becomes an important topic to be solved in the industry.
Disclosure of Invention
The invention provides a method for obtaining a nonlinear hybrid hardening model of a welding material, which aims to solve the technical problem that the nonlinear hybrid hardening model of the welding material cannot accurately represent the thermoelastic-plastic stress-strain relation of the welding material.
In order to solve the technical problems, the invention provides a method for obtaining a nonlinear hybrid hardening model of a welding material, which comprises the following steps:
s1, carrying out isothermal multistage cyclic tension-compression test on a welding material to obtain a stress-strain curve of the welding material at a set temperature;
S2, extracting stress-strain curves of each cycle, and respectively calculating yield strength sigma yn, maximum tensile stress sigma Fmaxn and yield strength sigma Rn of a compression stage of each cycle, wherein the average value of the yield strength sigma yn of the tensile stage of all the cycles is used as yield strength sigma y of the tensile stage of the welding material, the average value of the maximum tensile stress sigma Fmaxn of all the cycles is used as maximum tensile stress sigma Fmax of the welding material, the average value of the yield strength sigma Rn of the compression stage of all the cycles is used as yield strength sigma R of the compression stage of the welding material, the number of stages of the isothermal multistage cyclic tensile-compression test is more than or equal to 2, and the number of cycles n of each stage is more than or equal to 2;
S3, according to the formula The internal stress < sigma > is calculated according to the formulaCalculating a follow-up hardening model proportion P Kin, and calculating an isotropic hardening model proportion P Iso according to a formula P Iso＝1-PKin;
s4, fitting the A-F nonlinear follow-up hardening model by adopting data of a first cyclic stress-strain curve, and determining model parameters of the A-F nonlinear follow-up hardening model;
S5, taking P KinX+PIso Y as a nonlinear hybrid hardening model of the welding material, wherein X represents an A-F nonlinear follow-up hardening model after parameters are determined, and Y represents an isotropic hardening model.
Optionally, step S5 further includes the following steps:
S6, simulating isothermal multistage cyclic tension-compression test based on an elastoplastic finite element method, comparing simulated and actually measured stress-strain curves, and verifying the accuracy of the nonlinear hybrid hardening model of the welding material.
Optionally, step S6 further includes the following steps:
And S7, repeating the steps S1-S6, and obtaining nonlinear hybrid hardening models of the welding materials at different set temperatures.
Optionally, the shape of the welding material is a round bar, two ends of the welding material are clamping ends, and the clamping ends are provided with threads.
Optionally, the heating rate of the isothermal multistage cyclic tension-compression test is 5-15 ℃ per second, and the welding material is wrapped by heat-insulating cotton so as to ensure the temperature uniformity of parallel sections of the welding material.
Optionally, the parallel sections of the welding material are uniformly provided with 3 temperature sensors for acquiring temperature.
Optionally, the isothermal multistage cyclic tension-compression test has a multistage load not lower than 2 stages, the cyclic tension-compression times under each stage load not lower than 2 times, the strain rate not higher than 5×10 -4/s, and the first stage strain is larger than the elastic strain of the welding material at a set temperature to ensure that the welding material is plastically deformed in the test process, the subsequent strain is gradually increased, and the set maximum strain is required to ensure that the welding material is not bent and deformed in the compression process.
The method for acquiring the nonlinear hybrid hardening model of the welding material provided by the invention is based on hardening model criteria and a large number of experimental verification, derives the follow-up hardening model and the isotropic hardening model proportion calculation method, can quantitatively determine the isotropic hardening model proportion, and makes up the defects of the existing false management. By combining the A-F nonlinear follow-up hardening model and the isotropic hardening model, the established nonlinear hybrid hardening model can accurately calculate the thermoelastic plastic stress strain evolution process of the material in the welding process, and the accuracy of the welding numerical simulation result is improved.
Drawings
FIG. 1 is a flow chart of a method for obtaining a nonlinear hybrid hardening model of a welding material according to one embodiment of the invention.
Fig. 2 is a schematic parameter diagram of an isothermal multistage cyclic tension-compression test of a ferronickel-based superalloy provided in an embodiment of the present invention.
FIG. 3 is a cyclic stress-strain curve at 400℃for a ferronickel-based superalloy provided in accordance with an embodiment of the present invention.
FIG. 4 is a graph of maximum tensile stress versus cycle number for a ferronickel-based superalloy at 400 ℃ provided in an embodiment of the present invention.
Fig. 5a to 5C are graphs showing stress-strain curves calculated by different hardening models of the ferronickel-based superalloy provided by an embodiment of the present invention at 400 ℃ compared with actual measured values, fig. 5a is a graph showing the correspondence of an isotropic hardening model, fig. 5b is a graph showing the correspondence of a nonlinear follow-up hardening model, and fig. 5C is a graph showing the correspondence of a nonlinear hybrid hardening model provided by the present invention.
FIG. 6 is a graph of the ratio of the nickel-iron-based superalloy follow-up hardening model versus temperature according to an embodiment of the present invention.
FIGS. 7 a-7 h are graphs comparing predicted cyclic stress strain curves with measured values at room temperature, 200 ℃, 300 ℃, 400 ℃, 500 ℃, 600 ℃, 700 ℃ and 800 ℃ for a ferronickel-based superalloy according to an embodiment of the present invention.
Detailed Description
To further clarify the objects, advantages and features of the present invention, a method for obtaining a nonlinear hybrid hardening model of a welding material according to the present invention will be described in further detail with reference to the accompanying drawings. It should be noted that the drawings are in a very simplified form and are all to a non-precise scale, merely for convenience and clarity in aiding in the description of embodiments of the invention.
As shown in fig. 1, the present embodiment provides a method for obtaining a nonlinear hybrid hardening model of a welding material, the method comprising the steps of:
S1, carrying out isothermal multistage cyclic tension-compression test on the welding material, and obtaining a stress-strain curve of the welding material at a set temperature.
The ferronickel base alloy in the high-temperature alloy has good high-temperature mechanical property, high-temperature corrosion resistance and good creep property, is expected to play an important role in the ultra-supercritical thermal power unit, but the research on the constitutive relation of welding mechanics is still in a blank stage. The test and verification are mainly carried out on the ferronickel-based superalloy with a certain brand.
Optionally, the sample of the ferronickel-based superalloy is processed into a round bar sample, the length of the parallel section is 16mm, the diameter is 8mm, the length of the clamping end is 27mm, the screw thread specification is M16×1, and the transition arc radius of the clamping end and the parallel section is 20mm. Thus, the sample is convenient to fix, and the fixation of the temperature sensor and the extensometer is satisfied.
And (3) mounting the sample on a high-temperature fatigue testing machine, wherein an infrared heating furnace can be adopted to heat the sample to a set temperature, and the heating rate is 5-15 ℃ per second. The set temperature can be selected from room temperature to allowable temperature according to the requirement of the allowable temperature of the high-temperature fatigue testing machine. The 3 contact thermocouple temperature sensors can be uniformly arranged at the upper, middle and lower three positions of the parallel section of the sample, the sample is wrapped by using heat preservation cotton, the heating rate is set to 15 ℃ per second, and the heat preservation is carried out for 20 minutes, so that the temperature difference in the range of 16mm of the parallel section is ensured not to exceed +/-5 ℃.
The extensometer is fixed on a parallel section of a sample, and the strain rate is not higher than 5 multiplied by 10 -4/s and can be 2 multiplied by 10 -4/s by adopting strain control, so that isothermal multistage load cycle tensile and compression test is carried out. Fig. 2 is a schematic diagram of an isothermal multistage load cycle tensile compression test at 400 ℃ provided in this example, the multistage load designed for 3 stages of loads, the strain ranges being ± 0.5%, ±1.0% and ± 1.5%, respectively, with 10 cycles per stage.
The test procedure collects temperature, stress and strain data, and draws a stress-strain curve, and fig. 3 is a cyclic stress-strain curve at 400 ℃.
The relationship between the maximum tensile stress and the number of cycles can be obtained according to the stress-strain curve to determine the cycle hardening characteristics of the welding material. For example, the relationship between σ Fmaxn and the number of cycles can be drawn according to fig. 3, where σ Fmaxn is the maximum tensile stress at each cycle, as shown in fig. 4, the number of stages of the isothermal multistage cyclic tensile-compression test is equal to 3, and the number of cycles of each stage is n=10, and it can be seen from the graph that, at 400 ℃, the maximum tensile stress gradually increases with the increase of the number of cycles and strain, which indicates that the nickel-iron-based superalloy has an obvious strain hardening characteristic at 400 ℃.
S2, extracting stress-strain curves of each cycle, and respectively calculating yield strength sigma yn, maximum tensile stress sigma Fmaxn and yield strength sigma Rn of a compression stage of each cycle, wherein the average value of the yield strength sigma yn of the tensile stage of all the cycles is used as yield strength sigma y of the tensile stage of the welding material, the average value of the maximum tensile stress sigma Fmaxn of all the cycles is used as the maximum tensile stress sigma Fmax of the welding material, the average value of the yield strength sigma Rn of the compression stage of all the cycles is used as yield strength sigma R of the compression stage of the welding material, the number of stages of the isothermal multistage cyclic tensile-compression test is more than or equal to 2, and the number of cycles n of each stage is more than or equal to 2.
The calculation of σ yn、σFmaxn、σRn is of prior art and will not be described in detail here. The maximum tensile stress sigma Fmaxn is the corresponding tensile stress when the forward tensile strain reaches the set strain value under each cyclic load,Wherein m=the number of stages of isothermal multistage cyclic tension-compression test is n, m=3×10=30 times in fig. 4. The process of averaging σ y and σ R is similar to that of σ Fmax. The selection of the reverse strain offset determines the value of the yield strength sigma R in the compression stage, has important influence on the internal stress < sigma > and the follow-up hardening model proportion P Kin, and the range of the reverse strain offset value can be 0.02% -0.2%, and particularly can be 0.1% of the reverse strain offset.
S3, according to the formulaThe internal stress < sigma > is calculated according to the formulaThe slave hardening model proportion P Kin is calculated, and the equidirectional hardening model proportion P Iso is calculated according to the formula P Iso＝1-PKin.
The formula derivation process of this step is as follows:
The yield stress σ RI in the reverse compression phase can be expressed as:
|σRI|＝σFmax (1)
The yield stress σ RK for the reverse compression phase can be expressed as:
|σRK|＝2σy-σFmax (2)
Assuming the following hardening model ratio is P Kin, the yield strength σ R at the compression stage can be expressed as:
|σR|＝(1-PKin)|σRI|+PKin|σRK| (3)
Substituting the formula (1) and the formula (2) into the formula (3) to obtain:
let < σ > be the internal stress, < σ > can be expressed as:
thereby obtaining the following steps:
PIso＝1-PKin (7)
And S4, fitting the data of the first cyclic stress-strain curve to the A-F nonlinear follow-up hardening model, and determining model parameters of the A-F nonlinear follow-up hardening model.
The a-F nonlinear servo hardening model can be expressed as:
Wherein, In order to be able to rate of change of the plastic stress,In order to be an equivalent plastic strain,In order to provide a rate of change of plastic strain,In the event of a plastic stress,For equivalent plastic strain rate of change, h 1、h2 and k 2 are model parameters, and h 1、h2 and k 2 can be obtained by fitting measured stress-strain data from the first cycle.
S5, taking P KinX+PIso Y as a nonlinear hybrid hardening model of the welding material, wherein X represents an A-F nonlinear follow-up hardening model after parameters are determined, and Y represents an isotropic hardening model.
According to the method for obtaining the nonlinear hybrid hardening model of the welding material, provided by the embodiment, the following hardening model and the isotropic hardening model proportion calculating method are deduced based on hardening model criteria and a large number of experimental verification, so that the isotropic hardening model proportion can be quantitatively determined, and the defects of the existing false management are overcome. By combining the A-F nonlinear follow-up hardening model and the isotropic hardening model, the established nonlinear hybrid hardening model can accurately calculate the thermoelastic plastic stress strain evolution process of the material in the welding process, and the accuracy of the welding numerical simulation result is improved.
S6, simulating isothermal multistage cyclic tension-compression test based on an elastoplastic finite element method, comparing simulated and actually measured stress-strain curves, and verifying the accuracy of the nonlinear hybrid hardening model of the welding material.
The isothermal multistage cyclic tension-compression test of the nickel-iron-based superalloy at 400 ℃ can be simulated by finite element software, and the equivalent directional hardening model, the A-F nonlinear follow-up hardening model, the nonlinear hybrid hardening model and the actually measured cyclic stress strain curve are respectively compared, as shown in fig. 5 a-5C, the equivalent directional hardening model obviously overestimates the stress evolution, the A-F nonlinear follow-up hardening model underestimates the stress evolution, and the nonlinear hybrid hardening model is most consistent with the actually measured cyclic stress strain curve, so that the reliability of the method for obtaining the nonlinear hybrid hardening model of the welding material can be verified.
Optionally, the step S6 further comprises the step S7 of repeating the steps S1-S6 to obtain nonlinear hybrid hardening models of the welding materials at different set temperatures.
In order to further verify the accuracy of the method for obtaining the nonlinear hybrid hardening model of the welding material, as shown in fig. 7 a-7 h, isothermal multistage cyclic tension-compression tests at room temperature, 200 ℃, 300 ℃, 400 ℃, 500 ℃, 600 ℃, 700 ℃ and 800 ℃ are completed, the proportion of the follow-up hardening model and the proportion of the equidirectional hardening model at different temperatures are calculated, and fig. 6 is the relation between the proportion of the follow-up hardening model and the temperature, and as the temperature increases, the proportion of the follow-up hardening model shows a linear slow increasing trend. Meanwhile, according to the cyclic stress-strain curve, the parameters of the A-F follow-up hardening model at different temperatures are fitted, the isotropic hardening model of the isotropic hardening model proportion P Iso is combined and determined, a nonlinear hybrid hardening model is established, the cyclic stress-strain curve predicted at different temperatures is compared with an actual measurement value, and the stress-strain curve calculated by the nonlinear hybrid hardening model is very consistent with the actual measurement value.
In summary, the method for obtaining the nonlinear hybrid hardening model of the welding material provided by the invention derives the follow-up hardening model and the isotropic hardening model proportion calculation method based on the hardening model criterion and a large number of experimental verification, can quantitatively determine the isotropic hardening model proportion, and makes up the defects of the existing false management. By combining the A-F nonlinear follow-up hardening model and the isotropic hardening model, the established nonlinear hybrid hardening model can accurately calculate the thermoelastic plastic stress strain evolution process of the material in the welding process, and the accuracy of the welding numerical simulation result is improved.
The above description is only illustrative of the preferred embodiments of the present invention and is not intended to limit the scope of the present invention, and any alterations and modifications made by those skilled in the art based on the above disclosure shall fall within the scope of the present invention.
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