Method and model for predicting fatigue crack propagation rate of pipeline steel under hydrogen-mixed environments

Abstract

In a method for predicting the fatigue crack propagation rate of pipeline steel under a hydrogen-mixed environment, multiple compact tension test specimens of the pipeline steel to be measured are prepared. Then, fatigue crack propagation tests are performed on the compact tension test specimens under a nitrogen environment and under different hydrogen mixing ratio environments. The fatigue crack propagation rate and the corresponding stress intensity factor range are obtained and fitted to obtain a constant term. A fatigue crack propagation prediction model for the pipeline steel to be measured under a hydrogen-mixed environment is constructed, and finally, the fatigue crack propagation rate of the pipeline steel can be predicted by providing the hydrogen mixing ratio based on the fatigue crack propagation prediction model. [Effects] By conducting fatigue crack propagation tests in a nitrogen environment and under a few different hydrogen mixing ratios, a fatigue crack propagation prediction model for pipeline steel to be measured under a hydrogen mixing environment can be constructed, enabling prediction, effectively shortening the prediction test cycle, and ensuring the safety of test personnel and equipment.

Description

[Technical Field]

[0001] This invention belongs to the field of measurement of metallic materials, and more specifically, relates to a method and model for predicting the fatigue crack propagation rate of pipeline steel under a hydrogen-mixed environment. [Background technology]

[0002] As environmental pollution worsens, hydrogen energy is attracting widespread attention as a clean and efficient energy source, and transporting the produced hydrogen gas mixed with existing natural gas pipelines is considered the primary means of transporting hydrogen gas. When hydrogen mixed transport is carried out using natural gas pipelines, hydrogen molecules are adsorbed and dissociated, entering the pipeline steel as hydrogen atoms, diffusing, and accumulating at defects and stress concentrations, causing hydrogen embrittlement of the pipeline steel. On the other hand, pipelines are susceptible to fatigue failure due to fluctuations in internal pressure and changes in external loads. Furthermore, there is still no consensus on the selection of the hydrogen mixing ratio. Therefore, in order to determine an appropriate hydrogen mixing ratio and to ensure the safety and reliability of hydrogen mixed transport using natural gas pipelines, it is necessary to conduct fatigue performance tests of pipelines under different hydrogen mixing ratio environments. However, fatigue crack propagation rate tests of pipeline steel under multiple hydrogen mixing environments are costly and require long test cycles. If it were possible to predict fatigue performance in other hydrogen mixing environments based only on the results of fatigue crack propagation tests in a nitrogen environment and a small amount of hydrogen mixing environment, it would be of great significance to promote hydrogen mixed transport using natural gas pipelines.

[0003] CN103308381A discloses a fatigue crack propagation rate normalization prediction method that is simple and widely applicable, achieved by using a fatigue crack propagation rate curve at R=i to predict data for different stress ratios R≠i of the metal material under test, and by combining different types of stress intensity factors with energy as a control parameter. CN110411833A discloses a different frequency f iThe acceleration ratio A corresponding to different stress intensity factor ranges ΔK is calculated, the average value of the acceleration ratio is found, and then the frequency f is fitted using a quadratic polynomial. i This paper discloses a method for predicting crack propagation rates at different frequencies in a seawater corrosion environment by obtaining the relationship between the average acceleration ratio and the stress ratio. The two methods described above are applicable only to air-based and liquid corrosion environments; for gaseous corrosion environments, the model needs to be modified based on several parameters of the gaseous environment. Furthermore, the above methods depend on the change in stress ratio R and the load frequency f. i Since the focus is on predicting fatigue crack propagation rate based on changes in hydrogen content, it is not possible to predict changes in fatigue crack propagation rate due to changes in the test environment. Therefore, there is an urgent need to develop a method for predicting fatigue crack propagation rate in pipeline steel under different hydrogen mixing ratio environments. [Overview of the project]

[0004] In response to the shortcomings of the prior art, the present invention provides a method and model for predicting the fatigue crack propagation rate of pipeline steel under different hydrogen mixing ratio environments, in order to solve the problem that conventional prediction methods cannot be applied to gaseous corrosion environments.

[0005] According to the present invention, a method for predicting the fatigue crack propagation rate of pipeline steel under different hydrogen mixing ratio environments, comprising the following steps S1 to S3, S1: Multiple compact tension test specimens of the pipeline steel to be measured are prepared, and then fatigue crack propagation tests are performed on some of the compact tension test specimens under a nitrogen environment to determine the fatigue crack propagation rate under a nitrogen environment.

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[0006] Through the above-described technical means of the present invention, compared to the prior art, it is possible to construct a predictive model for fatigue crack propagation of pipeline steel to be measured simply by performing fatigue crack propagation tests in a nitrogen environment and a few different hydrogen mixing ratio environments, and to predict the fatigue crack propagation rate of pipeline steel at different hydrogen mixing ratios.

[0007] More preferably, before conducting the fatigue crack propagation test, multiple tension test specimens of the pipeline steel to be measured are prepared, the mechanical property parameters of the pipeline steel to be measured are obtained by tensile testing, and the average value of each mechanical property parameter is used as the input parameter for the fatigue crack propagation test.

[0008] More preferably, the number of tension test specimens is 3 to 5.

[0009] More preferably, in step S1, an initial crack with a length of 2 mm to 3 mm is introduced into the compact tension test piece.

[0010] More preferably, in step S1, a fatigue crack growth test is performed on at least one compact tension test piece in a nitrogen environment.

[0011] More preferably, in step S2, a fatigue crack growth test is performed on different compact tension test pieces in at least three hydrogen mixing ratio environments.

[0012] More preferably, in step S2, the hydrogen partial pressure P H is greater than 0.02 MPa.

[0013] More preferably, in steps S1 and S2, the stress intensity factor range ΔK is given by the following formula:

Equation

[0014] In another aspect of the present invention, a fatigue crack growth prediction model obtained by the above prediction method is provided.

[0015] From the above, the above technical means of the present invention have the following technical advantages compared with the prior art. 1. The present invention provides a method for predicting the fatigue crack growth rate of pipeline steel in a corrosive environment. By simply performing a fatigue crack growth test in a nitrogen environment and a small number of different hydrogen mixing ratio environments, a fatigue crack growth prediction model of pipeline steel to be measured in a hydrogen mixing environment can be constructed, the prediction of the fatigue crack growth rate of pipeline steel under different hydrogen mixing ratios can be realized, the test cycle of the prediction can be effectively shortened, and the safety of the test personnel and equipment can be guaranteed. 2. In particular, the present invention ensures predictive accuracy by optimizing the number of tests conducted under nitrogen and hydrogen mixed environments, while avoiding a decrease in safety due to an excessive number of tests. 3. Furthermore, by optimizing the range of hydrogen mixing ratios, the present invention avoids a decrease in test safety due to excessively high hydrogen mixing ratios. [Brief explanation of the drawing]

[0016] [Figure 1] This invention provides a method for predicting the fatigue crack propagation rate of pipeline steel under different hydrogen mixing ratio environments, as described in the embodiments of the present invention. [Figure 2] This is a schematic diagram of the structure of a compact tension test specimen provided in an embodiment of the present invention. [Figure 3] This is a schematic diagram of the structure of a tension test specimen provided in an embodiment of the present invention. [Figure 4] This is a curved surface diagram of a crack propagation prediction model for pipeline steel obtained in an embodiment of the present invention. [Figure 5] This is a comparison chart of measured data and predicted data under an environment with a hydrogen mixing ratio of 15% provided in an embodiment of the present invention. Figure 5(a) is a comparison chart of measured data and a prediction model, and Figure 5(b) is a comparison chart of measured data and a prediction curve. [Modes for carrying out the invention]

[0017] To further clarify the object, technical concept, and advantages of the present invention, the invention will be described in more detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are for illustrative purposes only and do not limit the invention.

[0018] As shown in Figure 1, according to one aspect of the present invention, a method for predicting the fatigue crack propagation rate of pipeline steel under different hydrogen mixing ratio environments is provided. The method specifically includes the following steps S1 to S3.

[0019] S1: Prepare multiple compact tension test specimens of the pipeline steel to be measured. Each compact tension test specimen will have the same basic parameters, such as thickness, width, and machined notch length. Next, some of the compact tension test specimens will be placed in a metal material environmental compatibility tester and fatigue crack propagation tests will be conducted under a nitrogen environment to determine the fatigue crack propagation rate under a nitrogen environment.

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[0020] S2: Fatigue crack propagation tests are performed on the remaining compact tension specimens under different hydrogen mixing ratio environments. Here, the total gas pressure, loading method, stress ratio, loading frequency, etc., are the same as in step S1. This determines the fatigue crack propagation rate under different hydrogen mixing ratio environments.

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[0021] S3: Substitute the constant terms obtained in steps S1 and S2 into equation (2) to construct a fatigue crack propagation prediction model for pipeline steel to be measured in a hydrogen-mixed environment. Based on this fatigue crack propagation prediction model, the fatigue crack propagation rate of the pipeline steel can be predicted by providing a hydrogen mixing ratio.

[0022] Specifically, the fatigue crack propagation rate of compact tension specimens under a hydrogen-mixed environment.

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[0023] In Equation (4),

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[0024] In Equation (4),

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[0025] Based on the above, the fatigue crack propagation rate under a hydrogen-mixed environment is:

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[0026] The method for predicting the fatigue crack propagation rate of pipeline steel under different hydrogen mixing ratios provided in this invention significantly reduces testing costs and improves efficiency because it can predict the fatigue crack propagation rate under other different hydrogen mixing ratios simply by conducting fatigue crack propagation tests under nitrogen and low-hydrogen mixing environments. Furthermore, fatigue crack propagation tests under hydrogen mixing environments are complicated to perform and have long test cycles. Prediction using a prediction model saves a great deal of time and allows for accurate and rapid prediction of the fatigue crack propagation rate under different hydrogen mixing ratios, providing a basis for selecting the hydrogen mixing ratio. In addition, hydrogen mixing environments are flammable and explosive, posing a serious safety hazard to testers and the environment. The prediction method provided in this invention can more reliably protect the safety of testers and equipment because it can predict the fatigue crack propagation rate under different hydrogen mixing ratios by performing only a small number of tests.

[0027] Furthermore, in step S1, before conducting the fatigue crack propagation test, multiple tension test specimens of the pipeline steel to be measured are prepared. These tension test specimens have the shape shown in Figure 3. Tensile tests are performed using these tension test specimens to obtain basic mechanical property parameters such as the modulus of elasticity, tensile strength, yield strength, and elongation ratio of the pipeline steel to be measured. The average value of each mechanical property parameter is used as an input parameter for the fatigue crack propagation test to calculate and output the crack length a.

[0028] Furthermore, the number of tension test specimens is between 3 and 5. This ensures the accuracy of the measurement of the basic mechanical property parameters of the pipeline steel being tested.

[0029] Furthermore, in step S1, as shown in Figure 2, the length of the pre-crack is related to the size of the compact tension specimen. In this invention, a pre-crack with a length L of 2 mm to 3 mm is introduced in the compact tension specimen. This eliminates the influence of the machined notch on subsequent crack propagation.

[0030] Furthermore, in step S1, a fatigue crack propagation test is performed on at least one compact tension specimen under a nitrogen environment. In step S2, fatigue crack propagation tests are performed on different compact tension specimens under at least three different hydrogen mixing ratio environments. Therefore, the number of compact tension specimens is at least four.

[0031] Furthermore, in step S2, the hydrogen mixing ratio is within the range of 5% to 30%. To avoid causing an increase in test hazards due to excessively high hydrogen content, several appropriate hydrogen mixing ratios can be selected within this range.

[0032] Furthermore, in steps S1 and S2, the stress intensity factor range ΔK is calculated by the following formula.

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[0033] According to another aspect of the present invention, a fatigue crack propagation prediction model obtained by the above prediction method is provided. The fatigue crack propagation rate of pipeline steel can be measured by providing a hydrogen mixing ratio.

[0034] The technical means provided by the present invention will be further described below with reference to specific examples.

[0035] The X65 pipeline steel, commonly used for natural gas transport, is used as the test subject, and the method for predicting the fatigue crack propagation rate of pipeline steel under a hydrogen-mixed environment provided in the present invention is used. This prediction method includes the following steps S1 to S3.

[0036] S1: Three dogbone-shaped tension test specimens were fabricated using X65 pipeline steel used in the test, and their tensile properties were measured. The final tensile property parameters are shown in Table 1. The average value of each parameter was used as the input parameter for the fatigue crack propagation test.

[0037] Table 1: Tensile properties of X65 pipeline steel [Table 1]

[0038] S2: Five compact tension test specimens were prepared and designated as specimens 1#, 2#, 3#, 4#, and 5#. Basic parameters such as specimen thickness, width, and machined notch length were measured. Under air conditions, the specimens were placed in a high-frequency fatigue testing machine to introduce a pre-crack of approximately 2 mm. In accordance with the national standard GB / T 34542.2-2018, "Hydrogen Storage and Transport Systems, Part 2: Test Methods for Metallic Materials and Hydrogen Environment Compatibility," a fatigue crack propagation rate test was performed on specimen 1# in a nitrogen environment. The total test pressure was set to 10 MPa, a sinusoidal load was used, the stress ratio was set to 0.1, and the test frequency was set to 1 Hz. The relationship between the fatigue crack propagation rate da / dN and the stress intensity factor range ΔK was obtained and fitted to the following equation using the Paris equation.

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[0039] S3: Fatigue crack propagation rate tests were performed on specimens 2#-4# under a hydrogen-mixed environment. The total test pressure was 10 MPa, and the hydrogen mixing ratios were 5%, 10%, and 20%, respectively. Other test parameters were the same as those for specimen 1#. The final fatigue crack propagation rate da / dN and hydrogen partial pressure P were determined. H Data sets were obtained for fatigue crack propagation rate (at 0.5, 1, and 2 MPa, respectively) and stress intensity factor range ΔK.

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[0040] Figure 4 is a surface view of this fitting model.

[0041] To verify the accuracy of the predictive model corresponding to equation (12), the fatigue crack propagation rate was measured for specimen 5# under a total pressure of 10 MPa and a hydrogen mixing ratio of 15%. Other test conditions were the same as those for specimens 1#-4#, and experimental data points were obtained.

[0042] Figure 5(a) shows the measured data distribution and the predicted model surface under conditions with a hydrogen mixing ratio of 15%, and the predicted surface shows very high consistency with the measured data points. Predicted crack propagation rates were measured for different stress intensity factor ranges ΔK using equation (10) and compared with the measured values ​​(Table 2 and Figure 5(b)). As can be seen from the comparison results, the predicted values ​​have a standard deviation of less than 10% from the measured values, indicating that this model can accurately and quickly predict fatigue crack propagation rates under different hydrogen mixing ratio conditions.

[0043] Table 2: Comparative statistical table of measured and predicted fatigue crack propagation rates under a 15% hydrogen mixture environment. [Table 2]

[0044] Those skilled in the art will know that these are merely preferred embodiments of the present invention and do not limit it, and any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for predicting the fatigue crack propagation rate of pipeline steel under different hydrogen mixing ratio environments, comprising the following steps S1 to S3, S1: Multiple compact tension test specimens of the pipeline steel to be measured are prepared, and then fatigue crack propagation tests are performed on some of the compact tension test specimens under a nitrogen environment to determine the fatigue crack propagation rate under a nitrogen environment. [Math 1] And the corresponding stress intensity factor range ΔK is obtained and fitted using formula (1) to obtain the constant term C １ and m １ Obtain, [Math 2] In the formula, a is the length of the crack, N is the number of stress cycles, and C １ and m １ These are fitting parameters in a nitrogenous environment, S2: Fatigue crack propagation rate of compact tension specimens in a hydrogen-mixed environment. [Math 3] It consists of the following two elements: [Math 4] During the ceremony, [Math 5] This is the fatigue crack propagation rate under a hydrogen-mixed environment. [Math 6] This represents the effect of a nitrogen environment on the fatigue crack propagation rate, and was calculated using equation (1). [Number 7] This is the fatigue crack propagation rate accelerated by hydrogen partial pressure, and was calculated using equation (5). [Number 8] During the ceremony, [Number 9] This is the transient-state hydrogen-enhanced fatigue crack growth rate. [Number 10] This is the steady-state hydrogen-enhanced fatigue crack growth rate, and C ２ , m ２ And n are fitting constants for the transient hydrogen-accelerated fatigue crack propagation rate, C ３ and m ３ This is the fitting constant for the steady-state hydrogen-accelerated fatigue crack propagation rate, Based on the above, the fatigue crack propagation rate under a hydrogen-mixed environment is: [Math 11] It is represented as, Fatigue crack propagation tests were performed on the remaining compact tension specimens under different hydrogen mixing ratios, and the fatigue crack propagation rates under different hydrogen mixing ratios were determined. [Math 12] and obtain the corresponding stress intensity factor range ΔK, and fit it by Equation (2) to obtain the constant term C ２ , C ３ , m ２ , m ３ and obtain m and n, [Number 13] In the formula, P Ｈ This is the partial pressure of hydrogen, S3: A prediction method characterized by substituting the constant terms obtained in steps S1 and S2 into formula (2) to construct a fatigue crack propagation prediction model for pipeline steel to be measured under a hydrogen mixed environment, and predicting the fatigue crack propagation rate of pipeline steel by providing a hydrogen mixing ratio based on the fatigue crack propagation prediction model.

2. The prediction method according to claim 1, characterized in that, in step S1, before conducting a fatigue crack propagation test, multiple tension test specimens of the pipeline steel to be measured are prepared, mechanical property parameters of the pipeline steel to be measured are obtained by tensile testing, and the average value of each mechanical property parameter is used as the input parameter for the fatigue crack propagation test.

3. The prediction method according to claim 2, characterized in that the number of tension test specimens is 3 to 5.

4. The prediction method according to claim 1, characterized in that in step S1, a pre-crack with a length of 2 mm to 3 mm is introduced into the compact tension test specimen.

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

6. The prediction method according to claim 1, characterized in that in step S2, fatigue crack propagation tests are performed on different compact tension test specimens under at least three different hydrogen mixing ratio environments.

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

8. In steps S1 and S2, the stress intensity factor range ΔK is given by the following formula: [Number 14] Calculated by, In the formula, P ｍａｘ The prediction method according to claim 1, characterized in that is the maximum load during measurement, B and W are the thickness and width of the compact tension test specimen, respectively, R is the stress ratio, and a is the length of the crack.

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