Method for comprehensively evaluating impact toughness of welded joint based on impact fracture crack propagation path

Abstract

The invention discloses a method for comprehensively evaluating the impact toughness of a welded joint based on an impact fracture crack propagation path, which comprises the following steps: designing different welding test parameters, and adopting different welding methods to obtain welding seam samples under multiple groups of test parameters; processing the welding sample into an impact sample, and carrying out impact toughness test on the center of a welding seam, a fusion line and an area outside the fusion line; observing an expansion path of the fracture section, and calculating a fracture crack expansion path distance; and according to each group of welding parameters and the corresponding fracture crack propagation path distance, a multiple linear regression model is obtained through multiple regression analysis, and the multiple linear regression model and the fracture crack propagation path distance are combined to obtain a weld joint impact toughness comprehensive evaluation model. According to the method, the impact performance of the welding sample piece can be rapidly and accurately evaluated, the impact toughness value of the welding joint obtained through different heat inputs is corrected, a large amount of trial and error cost caused by large discreteness of an impact experiment is reduced, and therefore the welding quality and efficiency are improved.

Description

Technical Field

[0001] The invention relates to a method for evaluating the impact toughness of a welded joint, and in particular to a method for comprehensively evaluating the impact toughness of a welded joint based on an impact fracture crack propagation path. Background Art

[0002] Whether the welded joints of ships, bridges and other structures can meet the toughness requirements during use, as well as the crack sensitivity of the material during welding, are all related to the overall quality and stability of the workpiece. Especially when facing service conditions such as low temperatures, whether the welded joints can meet their toughness indicators is particularly critical.

[0003] Generally, the practical significance of the material impact toughness index is to reveal the tendency of the material to become brittle, and it reflects the resistance of the metal material to external impact loads. The impact toughness of the welded joint is related to the metallographic structure, chemical composition, physical properties, welding process, etc. of the material itself, so the impact test becomes an indispensable means to inspect the metallurgical quality of the welded joint. Usually, the impact toughness of the weld is represented by the impact toughness value (αk) and the impact work (Ak). The tortuosity and length of the impact fracture path can be compared to judge the impact performance. The more tortuous the path and the longer the path length, the more obstacles the material encounters when it breaks, and it has a stronger ability to resist fracture. The existing evaluation method for the weldability of welded joints is mainly to control different welding parameters, take samples at the weld position, conduct impact tests on the samples at a certain temperature, and obtain their impact work to determine. However, due to the influence of the raw material's own properties, the sample preparation accuracy, and the sampling position, the discreteness of the impact test results is large, and the impact toughness is mainly determined by the unit cross-sectional area, and the impact toughness of the sample cannot be comprehensively evaluated, and the results have a certain degree of randomness. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to propose a method for comprehensively evaluating the impact toughness of welded joints based on the impact fracture crack propagation path, by adopting the fracture crack propagation path identification technology and the component multivariate linear regression analysis model, so as to achieve accurate evaluation of the impact performance of the sample.

[0005] Technical solution: The present invention includes the following steps: designing different welding test parameters, adopting different welding methods, and obtaining weld specimens under multiple groups of test parameters; processing the welding specimens into impact specimens, and performing impact toughness tests on the weld center, fusion line and the area outside the fusion line; observing the expansion path of the fracture section, and calculating the fracture crack expansion path distance; according to each group of welding parameters and the corresponding fracture crack expansion path distance, a multivariate linear regression model is obtained through multivariate regression analysis, and the multivariate linear regression model is combined with the fracture crack expansion path distance to obtain a comprehensive evaluation model for weld impact toughness.

[0006] The impact toughness of the weld is determined by the distance of the impact fracture crack propagation path and the cooling rate.

[0007] The comprehensive evaluation model of weld impact toughness is:

[0008] Y=A / L+B*t 8 / 5 +C

[0009] Among them, Y is the impact toughness, L is the distance of the crack propagation path of the impact fracture, A and B are regression coefficients, C is the random error variable, t 8 / 5 For cooling time.

[0010] The calculation model of the cooling time is:

[0011] t 8 / 5 =(E / δ) 2 / (4πλcρ)[1 / (500-T0) 2 -1 / (800-T0) 2 ]

[0012] Among them, E is the welding line energy, T0 is the initial temperature of the welding sample, δ is the test plate thickness, λ is the thermal conductivity, c is the specific heat capacity, and ρ is the density.

[0013] The multivariate linear regression model determines the regression coefficient according to the distance of the impact fracture crack propagation path corresponding to each group of welding parameters, thereby establishing the multivariate linear regression model.

[0014] The welding test parameters include welding method, welding voltage, welding current and welding speed.

[0015] The welding specimen is processed into a V-notch Charpy impact specimen.

[0016] The expansion path of the fracture section is observed using a microscope, and an impact fracture expansion path diagram is extracted based on the fracture section morphology.

[0017] The welding method includes gas shielded welding and argon arc welding.

[0018] The fracture crack propagation path distance is calculated using Image-Pro software.

[0019] Beneficial effects: The present invention has the following advantages:

[0020] (1) The present invention can realize rapid and accurate evaluation of the impact performance of welding samples, and calibrate the impact toughness values ​​of welding joints obtained with different heat inputs, so as to reduce the large trial and error costs caused by the large discreteness of impact tests, thereby improving welding quality and efficiency;

[0021] (2) By establishing the correlation between welding parameters, fracture crack path distance and joint impact toughness value, the impact toughness of the welded joint can be comprehensively and accurately evaluated, thereby reversely modifying parameters such as welding heat input so that the weld impact toughness value meets the technical index requirements, which can bring great convenience to engineering applications. DETAILED DESCRIPTION

[0022] The present invention will be further described below.

[0023] The method for comprehensively evaluating the impact toughness of welded joints based on the impact fracture crack propagation path of the present invention is to design welding parameters with different parameters, conduct welding tests, and obtain corresponding welding samples; prepare impact samples for each set of parameters, observe the impact fracture morphology through a 3D microscope, and extract the extension path of the fracture section, draw an impact fracture extension path diagram using PS software, calculate the fracture crack propagation path distance based on Image-Pro software, establish a multivariate regression analysis model based on the crack propagation path length corresponding to each group of welding heat inputs, combine the multivariate linear regression analysis model with the sample impact performance (impact work) obtained in the experiment, and obtain a comprehensive evaluation model for the impact performance of the welded sample. The present invention can realize a rapid and accurate evaluation of the impact performance of the welded sample, correct the impact toughness value of the welded joint obtained with different heat inputs, so as to reduce the large trial and error costs caused by the large discreteness of the impact test, thereby improving the welding quality and efficiency. Specifically comprising the following steps:

[0024] S1. Design different welding test parameters, adopt gas shielded welding and argon arc welding methods, and obtain weld specimens under multiple groups of test parameters; the welding parameters are welding method, welding voltage, welding current and welding speed, that is, obtain specimens under different welding methods, welding voltages, welding currents and welding speeds, and conduct impact tests to obtain the fracture crack propagation path and calculate the actual distance.

[0025] S2. Process the welding specimen into a V-notch Charpy impact specimen with a size of 10×10×55 mm, and perform impact toughness tests on the weld center, fusion line, and 1 mm outside the fusion line;

[0026] S3. The expansion path of the fracture section of the welding specimen was observed by 3D microscope. The fracture section morphology was extracted by PS software to obtain the impact fracture expansion path diagram. Image-Pro software was used to calculate the fracture crack expansion path distance.

[0027] S4. According to each set of welding parameters and the corresponding fracture crack propagation path distance, a multivariate linear regression equation is obtained through multivariate regression analysis. The multivariate linear regression model is combined with the fracture crack propagation path distance to obtain a comprehensive evaluation model for weld impact toughness. The weld impact performance is determined by the impact fracture crack propagation path distance, cooling rate, etc.:

[0028] Y=A / L+B*t 8 / 5 +C

[0029] Among them, Y is the impact toughness, L is the distance of the crack propagation path of the impact fracture, A and B are regression coefficients, C is the random error variable, t 8 / 5 For the cooling time, the relationship between welding heat input and cooling time is established, and the calculation model of cooling time is:

[0030] t 8 / 5 =(E / δ) 2 / (4πλcρ)[1 / (500-T0) 2 -1 / (800-T0) 2 ]

[0031] Among them, E is the welding line energy, T0 is the initial temperature of the welding sample, δ is the test plate thickness, λ is the thermal conductivity, c is the specific heat capacity, and ρ is the density.

[0032] According to the distance of the impact fracture crack propagation path corresponding to each group of welding parameters, the regression coefficient is determined to establish a multivariate linear regression model.

[0033] S5. Input the welding parameters into the comprehensive evaluation model to obtain the corresponding weld impact toughness value.

[0034] Example

[0035] (1) Through welding tests, the corresponding heat input, fracture crack propagation path distance, etc. under different welding parameters are obtained.

[0036] Various welding methods, such as gas shielded welding, argon arc welding, etc., are tested with different welding parameters for each welding method, and the cooling time of the weld is recorded and calculated.

[0037] The welding parameters are welding method, welding voltage, welding current and welding speed, that is, samples are obtained under different welding methods, welding voltages, welding currents and welding speeds, and impact tests are carried out to obtain the fracture crack propagation path and calculate the actual distance.

[0038] Submerged arc welding was performed on 48mm thick Q690qE steel with a welding current of 590-630A, a welding voltage of 28-31V, welding speeds of 65-70cm / min, 50-52cm / min and 42-45cm / min, and the calculated line energy values ​​were 18.0kJ / cm, 27.9kJ / cm and 40.3kJ / cm, respectively. The crack propagation paths of the fractures were measured and their lengths L were 12.1mm, 13.0mm and 14.3mm, respectively.

[0039] (2) According to the experimental data of each set of welding parameters, the cooling time t 8 / 5 Calculation.

[0040] The formula used in the calculation is:

[0041] t 8 / 5 =(E / δ) 2 / (4πλcρ)[1 / (500-T0) 2 -1 / (800-T0) 2 ]

[0042] Among them, E is the welding line energy, T0 is the initial temperature of the welding sample, δ is the test plate thickness, λ is the thermal conductivity, c is the specific heat capacity, and ρ is the density.

[0043] Combined with the calculation results of step (1), it is known that the interlayer temperature is 260°C, and λ=0.29J / (cm·s·°C), c ρ =6.7J / (cm3·℃), substituting the above value into its t 8 / 5 Formula, can be used to obtain t at different line energies 8 / 5 They are 22.9s, 35.5s and 51.3s respectively.

[0044] (3) Based on the fracture crack propagation path distance L and cooling time t obtained above 8 / 5 , a comprehensive evaluation model is obtained through multivariate regression analysis. The comprehensive evaluation model of impact toughness is:

[0045] Y=A / L+B*t 8 / 5 +C

[0046] Among them, Y is the impact toughness, L is the distance of the impact fracture crack propagation path, A and B are regression coefficients, and C is the random error variable. The impact toughness value is taken as the dependent variable Y, and the cooling time and the impact fracture crack propagation path distance are taken as independent variables.

[0047] According to the calculations in steps (1) and (2), we can get L and t 8 / 5 The average impact energy of the weld obtained by the three known process parameters is 202J, 209J and 127J respectively. The data is fitted, and the regression coefficients A and B, as well as the random error variable value C, are determined by multiple regression analysis to obtain a comprehensive evaluation model for impact fracture impact toughness. In the process of determining the regression coefficient, the Y value should be the mode of a large number of experimental results under each parameter to reduce the randomness of the experimental results. After obtaining the multiple regression analysis model, the regression equation should be analyzed for significance, and the model confidence level should be guaranteed to be above 95% through T test and F test.

[0048] (4) The comprehensive evaluation value of the impact fracture toughness is verified with the actual experimental results to verify that the model is more scientific.

Claims

1. A method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture, characterized in that: The following steps are involved: Different welding test parameters were designed, and different welding methods were adopted to obtain weld specimens under multiple groups of test parameters. The weld specimens were processed into impact specimens, and impact toughness tests were carried out on the weld center, fusion line and the area outside the fusion line. The expansion path of the fracture section was observed, and the fracture crack expansion path distance was calculated. According to each group of welding parameters and the corresponding fracture crack expansion path distance, a multivariate linear regression model was obtained through multivariate regression analysis, and the multivariate linear regression model was combined with the fracture crack expansion path distance to obtain a comprehensive evaluation model for weld impact toughness.

2. A method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture according to claim 1, characterized in that: The impact toughness of the weld is determined by the distance of the impact fracture crack propagation path and the cooling rate.

3. The method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture according to claim 2, characterized in that: The comprehensive evaluation model of weld impact toughness is: Y=A / L+B*t 8 / 5 +C Among them, Y is the impact toughness, L is the distance of the crack propagation path of the impact fracture, A and B are regression coefficients, C is the random error variable, t 8 / 5 For cooling time.

4. The method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture according to claim 3, characterized in that: The calculation model of the cooling time is: t 8 / 5 =(E / d) 2 / (4plcρ)[1 / (500-T0) 2 -1 / (800-T0) 2 ] Among them, E is the welding line energy, T0 is the initial temperature of the welding sample, δ is the test plate thickness, λ is the thermal conductivity, c is the specific heat capacity, and ρ is the density.

5. The method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture according to claim 1, characterized in that: The multivariate linear regression model determines the regression coefficient according to the distance of the impact fracture crack propagation path corresponding to each group of welding parameters, thereby establishing the multivariate linear regression model.

6. The method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture according to claim 1, characterized in that: The welding test parameters include welding method, welding voltage, welding current and welding speed.

7. The method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture according to claim 1, characterized in that: The welding specimen is processed into a V-notch Charpy impact specimen.

8. The method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture according to claim 1, characterized in that: The expansion path of the fracture section is observed using a microscope, and an impact fracture expansion path diagram is extracted based on the fracture section morphology.

9. The method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture according to claim 1, characterized in that: The welding method includes gas shielded welding and argon arc welding.

10. The method for comprehensively evaluating the impact toughness of a welded joint based on the crack propagation path of an impact fracture according to claim 8, characterized in that: The fracture crack propagation path distance is calculated using Image-Pro.