Method and model for predicting fatigue crack growth rate of pipeline steel in hydrogen-doped environment

Abstract

The invention belongs to the field of metal material testing, and particularly discloses a method and a model for predicting the fatigue crack growth rate of pipeline steel in a hydrogen-doped environment, and the prediction method comprises the following steps: processing a plurality of compact tensile samples of the pipeline steel to be tested, and then performing fatigue crack growth test on the compact tensile samples in a nitrogen environment and environments with different hydrogen doping ratios to obtain the fatigue crack growth rate of the pipeline steel. The fatigue crack propagation rate and the corresponding stress intensity factor range are obtained and fitted to obtain a constant term, so that a fatigue crack propagation prediction model of the to-be-detected pipeline steel in the hydrogen-doped environment is established, and finally the fatigue crack propagation rate of the pipeline steel can be predicted according to the hydrogen doping ratio given by the fatigue crack propagation prediction model. According to the method, the fatigue crack propagation prediction model of the to-be-tested pipeline steel in the hydrogen-doped environment can be established only by carrying out the fatigue crack propagation test in the nitrogen environment and a small number of environments with different hydrogen doping ratios, so that prediction is realized, the test period of prediction is effectively shortened, and the safety of testers and equipment can be ensured.

Description

Technical Field

[0001] This application belongs to the field of metal material testing, and more specifically, relates to a method and model for predicting the fatigue crack growth rate of pipeline steel in a hydrogen-doped environment. Background Art

[0002] With the increasing environmental pollution, hydrogen energy has attracted wide attention as a clean and efficient energy source. Mixing the produced hydrogen into the existing natural gas pipelines for transportation is considered an excellent means of transporting hydrogen. When the natural gas pipeline is used for hydrogen-hybrid transportation, on the one hand, hydrogen molecules enter the pipeline steel in the form of hydrogen atoms through adsorption and dissociation, diffuse and accumulate at defects and stress concentrations, which will cause hydrogen embrittlement failure of the pipeline steel; on the other hand, the pipeline is prone to fatigue failure due to the fluctuation of internal pressure and the change of external load. In addition, there is currently no conclusion on the selection of the hydrogen doping ratio. Therefore, in order to determine the appropriate hydrogen doping ratio and ensure the safety and reliability of the hydrogen-hybrid transportation of natural gas pipelines, it is necessary to conduct fatigue performance tests on pipelines in different hydrogen doping ratio environments. However, testing the fatigue crack growth rate of pipeline steel in multiple hydrogen-doped environments is costly and has a long test cycle. If the fatigue performance in other hydrogen-doped environments can be predicted only through the fatigue crack growth test results in a nitrogen environment and a small amount of hydrogen-doped environments, it has important engineering significance for promoting the operation of hydrogen-hybrid transportation of natural gas pipelines.

[0003] CN103308381A discloses a fatigue crack growth rate normalization prediction method. This method realizes the normalization prediction of data under different stress ratios R≠i of the metal material to be tested by the fatigue crack growth rate curve under R = i, and combines different types of stress intensity factors with energy as the control parameter, having the characteristics of simple method and wide application range. CN110411833A discloses a method for predicting the crack growth rate at different frequencies in a seawater corrosion environment. This method calculates the acceleration ratio A corresponding to different stress intensity factor ranges Δ i under different frequencies f K to obtain the average value of the acceleration ratio, and then obtains the relationship between the frequency f i and the average value of the acceleration ratio through quadratic polynomial fitting, thereby predicting the crack growth rate at other frequencies in the seawater corrosion environment. The above two methods only target the air medium and liquid corrosion environments. For the gas corrosion environment, the model needs to be corrected according to some parameters of the gas environment. In addition, the above methods target the prediction of the fatigue crack growth rate due to the change of stress ratio R and the change of loading frequency f i and cannot guide the prediction of the change of the fatigue crack growth rate caused by the change of the test environment. Therefore, it is urgent to develop a method for predicting the fatigue crack growth rate of pipeline steel in different hydrogen doping ratio environments. Summary of the Invention

[0004] In view of the deficiencies of the prior art, the present application provides a method and a prediction model for predicting the fatigue crack growth rate of pipeline steel in environments with different hydrogen doping ratios, aiming to solve the problem that existing prediction methods cannot be applied to gas corrosion environments.

[0005] A method for predicting the fatigue crack growth rate of pipeline steel in environments with different hydrogen doping ratios provided by the present application specifically includes: S1 Process multiple compact tension specimens of the pipeline steel to be tested, and then conduct fatigue crack growth tests on some of the compact tension specimens in a nitrogen environment to obtain the fatigue crack growth rate in the nitrogen environment and the corresponding stress intensity factor range Δ K , and use formula (1) for fitting to obtain the constant terms C 1 and m 1 : (1) In the formula, a is the crack length, N is the number of stress cycles, C 1 and m 1 are fitting parameters in the nitrogen environment; S2 Conduct fatigue crack growth tests on the remaining compact tension specimens in environments with different hydrogen doping ratios to obtain the fatigue crack growth rates in environments with different hydrogen doping ratios and the corresponding stress intensity factor range Δ K， and use formula (2) for fitting to obtain the constant terms C 2 , C 3 , m 2 , m 3 and n : (2) In the formula, P H is the hydrogen partial pressure; S3 Substitute the constant terms obtained in step S1 and step S2 into formula (2) to establish a fatigue crack growth prediction model for the pipeline steel to be tested in a hydrogen-doped environment. According to the fatigue crack growth prediction model, given the hydrogen doping ratio, the fatigue crack growth rate of the pipeline steel can be predicted.

[0006] Through the above technical solution conceived in this application, compared with the prior art, since this application only needs to carry out fatigue crack growth tests in a nitrogen environment and a few environments with different hydrogen doping ratios, a fatigue crack growth prediction model for the pipeline steel to be tested can be established, and then the prediction of the fatigue crack growth rate of the pipeline steel under different hydrogen doping ratios can be realized.

[0007] As a further preference, in step S1, before carrying out the fatigue crack growth test, tensile specimens of multiple pipeline steels to be tested are processed, and the mechanical property parameters of the pipeline steels to be tested are obtained through tensile tests, and the average value of each mechanical property parameter is used as the input parameter for the fatigue crack growth test.

[0008] As a further preference, the number of tensile specimens is 3 to 5.

[0009] As a further preference, in step S1, a crack with a length of 2 mm to 3 mm is prefabricated on the compact tensile specimen.

[0010] As a further preference, in step S1, at least one compact tensile specimen is subjected to a fatigue crack growth test in a nitrogen environment.

[0011] As a further preference, in step S2, fatigue crack growth tests are carried out on different compact tensile specimens in at least three hydrogen doping ratio environments.

[0012] As a further preference, in step S2, the hydrogen partial pressure P H is greater than 0.02 MPa.

[0013] As a further preference, in steps S1 and S2, the stress intensity factor range Δ K is calculated using the following formula: (3) In the formula, is the maximum load during the test, B 、 W are the thickness and width of the compact tensile specimen respectively, R is the stress ratio, a is the crack length.

[0014] According to another aspect of this application, a fatigue crack growth prediction model obtained by using the above prediction method is provided.

[0015] Generally speaking, compared with the prior art, the above technical solution conceived in this application mainly has the following technical advantages: 1. The present application provides a method for predicting the fatigue crack growth rate of pipeline steel in a corrosive environment. Only fatigue crack growth tests need to be carried out in a nitrogen environment and a few environments with different hydrogen doping ratios, and then a fatigue crack growth prediction model of the pipeline steel to be tested in a hydrogen-doped environment can be established, so as to realize the prediction of the fatigue crack growth rate of pipeline steel under different hydrogen doping ratios, effectively shortening the test cycle of the prediction and ensuring the safety of test personnel and equipment. 2. In particular, the present application optimizes the number of tests in the nitrogen environment and the hydrogen-doped environment, which can avoid the decrease in safety caused by excessive test times while ensuring the prediction accuracy. 3. In addition, the present application optimizes the range of the hydrogen doping ratio, which can avoid the decrease in test safety caused by too high a hydrogen doping ratio. Description of the Drawings

[0016] Figure 1 is a method for predicting the fatigue crack growth rate of pipeline steel in an environment with different hydrogen doping ratios provided by an embodiment of the present application; Figure 2 is a schematic structural diagram of a compact tension specimen provided by an embodiment of the present application; Figure 3 is a schematic structural diagram of a tensile specimen provided by an embodiment of the present application; Figure 4 is a surface diagram of a pipeline steel crack growth prediction model obtained by an embodiment of the present application; Figure 5 is a comparison diagram of measured data and predicted data in an environment with a hydrogen doping ratio of 15% provided by an embodiment of the present application, where (a) is a comparison diagram of measured data and the prediction model, and (b) is a comparison diagram of measured data and the prediction curve. Detailed Embodiments

[0017] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0018] As Figure 1 shown, according to one aspect of the present application, a method for predicting the fatigue crack growth rate of pipeline steel in an environment with different hydrogen doping ratios is provided, specifically including: S1 Process a plurality of compact tension specimens of the pipeline steel to be tested. The basic parameters such as the thickness, width, and machined notch length of each compact tension specimen are the same. Then place some of the compact tension specimens on a metal material environment compatibility testing machine and conduct fatigue crack growth tests in a nitrogen environment to obtain the fatigue crack growth rate and the corresponding stress intensity factor range Δ K, and the constant term is obtained by fitting using formula (1) C 1 and m 1 : (1) In the formula, a is the crack length, N is the number of stress cycles, C 1 and m 1 are the fitting parameters in the nitrogen environment; S2 conducts fatigue crack growth tests on the remaining compact tension specimens in environments with different hydrogen doping ratios, where the total gas pressure, loading method, stress ratio, loading frequency, etc. are the same as those in step S1, so as to obtain the fatigue crack growth rates and the corresponding stress intensity factor range Δ K， Taking as the Z-axis, the hydrogen partial pressure P H (converted by the hydrogen doping ratio and the total pressure) as the X-axis, and the stress intensity factor range Δ K as the Y-axis, and the constant term is obtained by fitting using formula (2) through the non-linear least squares method C 2 , C 3 , m 2 , m 3 and n : (2) In the formula, P H is the hydrogen partial pressure; S3 substitutes the constant terms obtained in step S1 and step S2 into formula (2) to establish a fatigue crack growth prediction model for the pipeline steel to be measured in the hydrogen-doped environment. According to the fatigue crack growth prediction model, the fatigue crack growth rate of the pipeline steel can be predicted by giving the hydrogen doping ratio.

[0019] Specifically, in the hydrogen-doped environment, the fatigue crack growth rate of the compact tension specimen consists of the following two aspects: (4) In the formula, is the fatigue crack growth rate in the hydrogen-doped environment, is the effect of the nitrogen environment on the fatigue crack growth rate, which is calculated by formula (1), is the fatigue crack growth rate accelerated by the hydrogen partial pressure, which is calculated by formula (4), (5) In the formula, is the transient hydrogen-induced fatigue crack growth rate, is the steady-state hydrogen-induced fatigue crack growth rate; The first term in formula (4) is expressed as: (6) In the formula a 1 、m 2 、c 1 and d 1 are constants, P H is the hydrogen partial pressure, Q is the activation energy, v is the partial molar volume of hydrogen in the metal, σ b is the stress at the critical distance from the crack tip, R is the gas constant, T is the thermodynamic temperature. Under the condition of the same temperature, (-Q+V σ b ) / (RT) is simplified and extracted as a constant term. Therefore, the first term is simplified to: (7) In the formula, C 2 , m 2 and n are the fitting constants of the transient hydrogen-induced fatigue crack growth rate; The second term in formula (4) is expressed as: (8) In the formula, a 2 、m 3 、c 2 and d 2 are constants. The definitions of other parameters are the same as those in formula (5). The change of hydrogen pressure has little effect on the steady-state hydrogen-induced fatigue crack growth rate. Therefore, the parameter d 2 is set to 0. The second term is simplified to: (9) In the formula, C 3 and m 3 are the fitting constants of the steady-state hydrogen-promoted fatigue crack growth rate.

[0020] In summary, the fatigue crack growth rate in a hydrogenated environment is expressed as: (2).

[0021] The prediction method for the fatigue crack growth rate of pipeline steel in different hydrogen doping ratio environments provided by this application can predict the fatigue crack growth rate under other different hydrogen doping ratios only by conducting fatigue crack growth tests in a nitrogen environment and a small amount of hydrogen doping environments, which greatly saves the test cost and improves the efficiency. At the same time, considering that the operation steps of the fatigue crack growth test in a hydrogenated environment are cumbersome and the test cycle is long, predicting through the prediction model saves a lot of time and can accurately and quickly predict the fatigue crack growth rate in different hydrogen doping ratio environments, providing a basis for the selection of the hydrogen doping ratio. In addition, the hydrogen doping gas environment is flammable and explosive, posing great safety hazards to test personnel and the environment. The prediction method provided by this application only needs to conduct a small number of tests to predict the fatigue crack growth rate in different hydrogen doping ratio environments, which can better ensure the safety of test personnel and equipment.

[0022] Further, in step S1, before conducting the fatigue crack growth test, multiple tensile specimens of the pipeline steel to be tested are processed. The tensile specimen has the shape as Figure 3 shown. Using this tensile specimen to conduct a tensile test to obtain basic mechanical property parameters such as the elastic modulus, tensile strength, yield strength, and elongation of the pipeline steel to be tested, and taking the average value of each mechanical property parameter as the input parameter for the fatigue crack growth test, which is used to calculate and output the crack length a .

[0023] Further, the number of tensile specimens is 3 to 5, so as to ensure the accuracy of the test of the basic mechanical property parameters of the pipeline steel to be tested.

[0024] Further, in step S1, as Figure 2 shown, the prefabricated crack length is related to the size of the compact tension specimen. In this application, a crack with a length L of 2 mm to 3 mm is prefabricated on the compact tension specimen, so as to eliminate the influence of the machining notch on the subsequent crack growth.

[0025] Further, in step S1, in a nitrogen environment, fatigue crack growth tests are conducted on at least one compact tension specimen. In step S2, fatigue crack growth tests are conducted on different compact tension specimens in at least three hydrogen doping ratio environments. Therefore, the number of compact tension specimens is at least four.

[0026] Further, in step S2, the hydrogen doping ratio is in the range of 5% to 30%. Several appropriate hydrogen doping ratios can be selected within this range to avoid an increase in the test risk factor due to excessive hydrogen content.

[0027] Further, in steps S1 and S2, the stress intensity factor range Δ K is calculated using the following formula: (3) In the formula, is the maximum load during the test, B , W are the thickness and width of the compact tension specimen respectively, R is the stress ratio, a is the crack length.

[0028] According to another aspect of the present application, a fatigue crack growth prediction model obtained by using the above prediction method is provided. Given the hydrogen doping ratio, the fatigue crack growth rate of pipeline steel can be predicted.

[0029] The following further illustrates the technical solutions provided by the present application according to specific embodiments.

[0030] Select the commonly used X65 pipeline steel for natural gas transportation as the verification object, and adopt the prediction method for the fatigue crack growth rate of pipeline steel in a hydrogen-doped environment provided by the present invention. This prediction method includes the following steps: S1: Machine 3 dog-bone-shaped tensile specimens on the X65 pipeline steel used in the test for tensile property testing. The final obtained tensile property parameters are shown in Table 1. Take the average value of each parameter as the input parameter for the fatigue crack growth test.

[0031] Table 1 Tensile properties of X65 pipeline steel

[0032] S2: Machine 5 compact tension specimens, namely specimen 1#, 2#, 3#, 4# and 5#, and measure the basic parameters such as the thickness, width, and machined notch length of the specimens. In an air environment, place the specimens on a high-frequency fatigue testing machine to prefabricate a crack of about 2 mm. According to the national standard GB / T 34542.2-2018 "Hydrogen storage and transportation system - Part 2: Test method for the compatibility of metal materials with hydrogen environment", carry out the fatigue crack growth rate test of specimen 1# in a nitrogen environment. The total test pressure is 10 MPa, sine wave loading is adopted, the stress ratio is 0.1, and the test frequency is 1 Hz to obtain the fatigue crack growth rate da / dN and the relationship with the stress intensity factor range Δ K . It is fitted by the Paris formula as: (10) That isC 1 = 2.511×10 -9 , m 1 = 3.030。

[0033] S3 carried out fatigue crack growth rate tests on specimens 2# - 4# under a hydrogen - doped environment. The total pressure of the test was 10 MPa, and the hydrogen - doping ratios were 5%, 10%, and 20% respectively. Other test parameters were the same as those of specimen 1#. Finally, the fatigue crack growth rate da / dN 、hydrogen partial pressure P H (which were 0.5, 1, and 2 MPa respectively) and the stress intensity factor range Δ K data sets. Taking the fatigue crack growth rate as the z - axis, the hydrogen partial pressure P H as the x - axis, and the stress intensity factor range Δ K as the y - axis, non - linear least - squares fitting was performed according to the following formula: (11) Finally, the fitting parameters C 2 = 1.128×10 -8 , C 3 = 1.494×10 -6 , m 2 = 3.486, m 3 = 2.176 and n = 0.183. The final fitting model is: (12) Figure 4 is the surface plot of this fitting model.

[0034] To verify the accuracy of the prediction model corresponding to formula (12), specimen 5# was specified to test the fatigue crack growth rate under a hydrogen - doping ratio of 15% and a total pressure of 10 MPa. Other test conditions were the same as those of specimens 1# - 4#, and the measured data points were obtained.

[0035] Under the environment with a hydrogen - doping ratio of 15%, the distribution of the measured data and the prediction model surface are as shown in Figure 5 (a). The prediction surface and the measured data points are in good agreement. According to formula (10), the predicted values of the crack growth rate under different stress intensity factor ranges Δ K were calculated and compared with the measured values, as shown in Table 2 and Figure 5As shown in Fig. (b). The comparison results show that the standard deviation of the predicted value relative to the measured value is within 10%, and the model can accurately and quickly predict the fatigue crack growth rate in environments with different hydrogen doping ratios.

[0036] Table 2 Comparison and verification statistical table of measured and predicted values of fatigue crack growth rate in an environment with a 15% hydrogen doping ratio

[0037] It is easy for those skilled in the art to understand that the above are only preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for predicting fatigue crack growth rate of pipeline steel under different hydrogen doping ratio environments, characterized in that: Specifically include: S1 processes multiple compact tensile specimens of pipeline steel to be tested, and then conducts fatigue crack growth tests on some compact tensile specimens in a nitrogen environment to obtain the fatigue crack growth rate in a nitrogen environment and the corresponding stress intensity factor range Δ K , and use formula (1) to fit to obtain the constant term C 1 and m 1: （1） In the formula, a is the crack length, N is the number of stress cycles, C 1 and m 1 is the fitting parameter in nitrogen environment; S2 carried out fatigue crack growth tests on the remaining compact tensile specimens under different hydrogen doping ratio environments to obtain the fatigue crack growth rates under different hydrogen doping ratio environments. and the corresponding stress intensity factor range Δ K， And use formula (2) to fit to get the constant term C 2, C 3. m 2, m 3 and n : （2） In the formula, P H is the hydrogen partial pressure; S3 substitutes the constant terms obtained in step S1 and step S2 into formula (2) to establish a fatigue crack growth prediction model for the pipeline steel to be tested in a hydrogen-doped environment. According to the fatigue crack growth prediction model, the fatigue crack growth rate of the pipeline steel can be predicted given the hydrogen doping ratio.

2. The prediction method according to claim 1, characterized in that: In step S1, before the fatigue crack growth test is performed, a plurality of tensile test specimens of the pipeline steel to be tested are processed, and the mechanical property parameters of the pipeline steel to be tested are obtained through the tensile test, and the average value of each mechanical property parameter is used as the input parameter of the fatigue crack growth test.

3. The prediction method according to claim 2, characterized in that: The number of tensile specimens is 3 to 5.

4. The prediction method according to claim 1, characterized in that: In step S1, a crack with a length of 2 mm to 3 mm is prefabricated on the compact tensile specimen.

5. The prediction method according to claim 1, characterized in that: In step S1, a fatigue crack growth test is performed on at least one compact tensile specimen in a nitrogen environment.

6. The prediction method according to claim 1, characterized in that: In step S2, fatigue crack growth tests are performed on compact tensile specimens with different hydrogen doping ratios under at least three conditions.

7. The prediction method according to claim 1, characterized in that: In step S2, the hydrogen partial pressure P H Greater than 0.02MPa.

8. The prediction method according to claim 1, characterized in that: In steps S1 and S2, the stress intensity factor range Δ K Use the following formula to calculate: （3） In the formula, is the maximum load during the test, B , W are the thickness and width of the compact tensile specimen, R is the stress ratio, a is the crack length.

9. A fatigue crack growth prediction model obtained by using the prediction method according to any one of claims 1 to 8.

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