Test method for evaluating hydrogen embrittlement sensitivity index of welded joint by using principal component analysis method

Abstract

The invention discloses a test method for evaluating a hydrogen embrittlement sensitivity index of a welded joint by using a principal component analysis method, and belongs to the technical field of material performance evaluation, and the test method comprises the following steps: S1, processing a pipeline welded joint into a notched round rod slow strain rate tensile sample, and carrying out a slow strain rate tensile test; s2, the pipeline welding joint is machined into a fracture toughness stepped notch CT sample, and a fracture toughness test is conducted; s3, calculating a specific performance loss rate in the hydrogen-doped environment, and carrying out standardization processing; s4, calculating to obtain a hydrogen embrittlement sensitivity evaluation index IPC; s5, performing scanning electron microscope analysis on the slow strain rate tensile fracture, and judging the risk level of hydrogen induced cracking of the sample; and S6, determining an IPC range for distinguishing different hydrogen induced cracking risk levels. According to the test method for evaluating the hydrogen embrittlement sensitivity index of the welded joint by using the principal component analysis method, the statistical significance of small sample data can be improved, and a more objective and comprehensive hydrogen embrittlement sensitivity evaluation system is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of material performance evaluation, in particular to a testing method for evaluating the hydrogen embrittlement sensitivity index of a welded joint by using a principal component analysis method. Background Art

[0002] As the core clean energy carrier for achieving the goal of carbon neutrality, hydrogen energy has shown great application potential in the fields of transportation, energy storage and industry. The global industrial chain is accelerating the large-scale application of high-pressure hydrogen storage equipment, hydrogen pipelines and hydrogen fuel cells. However, hydrogen can easily cause hydrogen-induced cracking of metal materials under high-pressure environments, especially in the area of ​​pipeline welded joints. Due to the uneven structure and performance of welded joints, as well as the influence of residual stress and various welding defects, hydrogen embrittlement failure is a common problem of pipeline steel welded joints in hydrogen environments. As a key parameter for quantifying the material's resistance to hydrogen damage, the hydrogen embrittlement sensitivity index relies on a single indicator (such as elongation loss rate or cross-sectional shrinkage loss rate) in traditional evaluation methods, which has significant limitations. A single indicator cannot characterize the synergistic effect of multi-dimensional performance degradation and is easily affected by fluctuations in test conditions. Summary of the Invention

[0003] The purpose of the present invention is to provide a test method for evaluating the hydrogen embrittlement sensitivity index of welded joints using principal component analysis. By using principal component analysis, the multi-dimensional performance loss rate in a hydrogen environment is innovatively integrated into a one-dimensional comprehensive index, while retaining the main information, eliminating redundancy and conflict between indicators, and establishing a relationship between the risk degree of hydrogen-induced cracking and performance indicators. This can improve the statistical significance of small sample data and obtain a more objective and comprehensive hydrogen embrittlement sensitivity evaluation system.

[0004] To achieve the above object, the present invention provides a test method for evaluating the hydrogen embrittlement sensitivity index of a weld joint using principal component analysis, comprising the following steps:

[0005] S1. Process the pipe weld joint into a notched round bar slow strain rate tensile specimen. Notches are processed in the base metal, weld, and heat-affected zone. Slow strain rate tensile tests are then performed in a hydrogen environment and an inert gas environment.

[0006] S2. Process the pipeline weld joint into a fracture toughness step-notch CT specimen, process notches in the base material, weld, and heat-affected zone, and conduct fracture toughness tests in hydrogen and inert gas environments respectively;

[0007] S3. Calculate the specific performance loss rate under the hydrogen-doped environment based on the data obtained in S1 and S2, and perform standardization on the specific performance loss rate;

[0008] S4. Calculate the hydrogen embrittlement sensitivity evaluation index I based on the standardized specific performance loss ratePC ;

[0009] S5. Perform scanning electron microscopy analysis on the slow strain rate tensile fracture to determine the risk level of hydrogen-induced cracking of the sample;

[0010] S6. Establishment I PC Relationship with hydrogen-induced cracking risk level, determine the I PC scope.

[0011] Preferably, in S1, sampling of the base material, weld and heat-affected zone machining notches is performed at 12, 3 and 6 o'clock positions of the pipeline welding to perform repeated tests to obtain the notch tensile strength NTS, the maximum displacement l1 of the extensometer and the cross-sectional area A1 of the fracture;

[0012] The calculation formula of elongation δ is as follows:

[0013]

[0014] Where l0 is the original gauge length, l1 is the maximum displacement of the extensometer during the stretching process;

[0015] The calculation formula of the cross-sectional reduction rate ψ is as follows:

[0016]

[0017] Among them, A0 is the original cross-sectional area of ​​the test section, and A1 is the cross-sectional area of ​​the fractured part of the test section.

[0018] Preferably, S2 obtains the fracture toughness value J of the weld joint.

[0019] Preferably, the specific performance loss rate I under the hydrogen-doped environment in S3 m The calculation formula is as follows:

[0020]

[0021] Among them, m0 is the specific performance of the sample in an inert gas environment, m H is the specific performance of the sample in a hydrogen-doped environment; when m is δ, I m is the elongation loss rate I δ ; When m is ψ, I m is the area reduction loss rate I ψ ; When m is NTS, I m is the notch tensile strength loss rate I NTS ; When m is J, I m is the fracture toughness loss rate I J ;

[0022] Specific performance loss rate I under hydrogen doping environmentm The calculation formula for normalization is as follows:

[0023]

[0024] Among them, Z m is the normalized specific performance loss rate, is the overall mean specific performance loss rate, σ m is the standard deviation of the overall specific performance loss rate.

[0025] Preferably, the dimension of the performance loss rate matrix B in S4 is a×b, where a is the number of samples, that is, the total number of conditions for different combinations of hydrogen doping environments and weld joint areas, and b is the number of performance loss rate types. The covariance matrix Cov(B) is calculated based on the performance loss rate matrix B, and its dimension is b×b. The calculation formula of the covariance matrix Cov(B) is as follows:

[0026]

[0027] Among them, B T is the transposed matrix of the performance loss rate matrix B, whose dimension is b×a;

[0028] The eigenvalue λ is calculated according to the equation det(Cov(B)-λI)=0 i , i=1,2,...,b, where λ1>λ2>...>λ b , det() is the determinant calculation of the matrix, I is the identity matrix, and λ i Substitute the homogeneous linear equations (Cov(B)-λ i I) v=0 to obtain the feature vector v i , and using the formula For the eigenvector v i Normalization is performed to obtain the normalized eigenvector u i ,in

[0029] According to I PCi =Bu i Obtain the hydrogen embrittlement sensitivity index I of different principal component directions PCi ;

[0030] according to Get the variance explanation rate β i , take β i >70% corresponding to I PCi As the final hydrogen embrittlement sensitivity evaluation index I PC .

[0031] Preferably, 10 areas are observed at a magnification of 1000 times for each fracture center in S5. If the number of areas corresponding to the cleavage fracture mode is greater than 5, the hydrogen-induced cracking level of the sample is considered to be high risk; if the number of areas corresponding to the ductile fracture mode is greater than 5, the hydrogen-induced cracking level of the sample is considered to be low risk; if the number of areas corresponding to the quasi-cleavage fracture mode is greater than 5 or the number of areas corresponding to the cleavage fracture mode, the ductile fracture mode, and the quasi-cleavage fracture mode are all less than 5, that is, the fracture of the sample is mainly in the quasi-cleavage fracture mode, and the hydrogen-induced cracking level of the sample is considered to be medium risk.

[0032] Preferably, in S6, PC When n2 is less than I PC <n3 corresponds to a medium risk of hydrogen induced cracking, and when n4<I PC The corresponding hydrogen-induced cracking level is high risk, where n1≤n2≤n3≤n4, n1 and n2 represent all I PC After the values ​​are sorted from large to small, the hydrogen-induced cracking risk level is at the junction of low risk and medium risk, where n1 corresponds to the sample hydrogen-induced cracking risk level of low risk, n2 corresponds to the sample hydrogen-induced cracking risk level of medium risk, n3 and n4 represent all I PC After the values ​​are sorted from large to small, the hydrogen-induced cracking risk level is at the junction of medium risk and high risk, where n3 corresponds to a medium risk hydrogen-induced cracking risk level for the sample, and n4 corresponds to a high risk hydrogen-induced cracking risk level for the sample.

[0033] Therefore, the present invention adopts the above-mentioned test method for evaluating the hydrogen embrittlement sensitivity index of welded joints using the principal component analysis method. The principal component analysis method is used to innovatively integrate the multi-dimensional performance loss rate in a hydrogen environment into a one-dimensional comprehensive indicator, while retaining the main information, eliminating the redundancy and conflict between the indicators, and establishing a relationship between the risk degree of hydrogen-induced cracking and the performance indicators. It can improve the statistical significance of small sample data and obtain a more objective and comprehensive hydrogen embrittlement sensitivity evaluation system.

[0034] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a flow chart of the present invention;

[0036] Figure 2 Schematic diagram of sampling for the notched slow strain rate tensile test of the X65 pipeline steel welded joint of the present invention;

[0037] Figure 3 Schematic diagram of the fracture toughness CT sample sampling of X65 pipeline steel welded joints of the present invention;

[0038] Figure 4 This invention IPC Relationship diagram with hydrogen-induced cracking risk level. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0040] Unless otherwise defined, technical or scientific terms used in the present invention shall have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs.

[0041] Example 1

[0042] like Figure 1 As shown, the present invention provides a test method for evaluating the hydrogen embrittlement sensitivity index of a weld joint using a principal component analysis method, comprising the following steps:

[0043] S1. Process the pipe weld joint into a notched round bar slow strain rate tensile specimen. Notches are processed in the base metal, weld, and heat-affected zone. Slow strain rate tensile tests are then performed in a hydrogen environment and an inert gas environment.

[0044] The specimen size and test procedure should meet the relevant requirements of GB / T34542.2-2018. Samples of the base material, weld and heat-affected zone machined notches should be taken at 12, 3 and 6 o'clock positions of the pipeline weld for repeated testing to obtain the notch tensile strength NTS, the maximum displacement l1 of the extensometer and the cross-sectional area A1 of the fracture.

[0045] The calculation formula of elongation δ is as follows:

[0046]

[0047] Where l0 is the original gauge length, l1 is the maximum displacement of the extensometer during the stretching process;

[0048] The calculation formula of the cross-sectional reduction rate ψ is as follows:

[0049]

[0050] Where A0 is the original cross-sectional area of ​​the test section, and A1 is the cross-sectional area of ​​the test section at the fracture. Each cross-sectional area must be measured at least three times, and the minimum value is used as the final value of A0 and A1.

[0051] S2. Process the pipeline weld joint into a fracture toughness step-notch CT specimen, and process notches in the base material, weld, and heat-affected zone. Perform fracture toughness tests in a hydrogen environment and an inert gas environment respectively. The specimen size and test procedure should meet the relevant requirements of GB / T34542.2-2018 to obtain the fracture toughness value J of the weld joint.

[0052] S3. Calculate the specific performance loss rate under the hydrogen-doped environment based on the data obtained in S1 and S2, and perform standardization on the specific performance loss rate;

[0053] Specific performance loss rate I under hydrogen-doped environment m The calculation formula is as follows:

[0054]

[0055] Among them, m0 is the specific performance of the sample in an inert gas environment, m H is the specific performance of the sample in a hydrogen-doped environment; when m is δ, I m is the elongation loss rate I δ ; When m is ψ, I m is the area reduction loss rate I ψ ; When m is NTS, I m is the notch tensile strength loss rate I NTS ; When m is J, I m is the fracture toughness loss rate I J ;

[0056] Specific performance loss rate I under hydrogen doping environment m The calculation formula for normalization is as follows:

[0057]

[0058] Among them, Z m is the normalized specific performance loss rate, is the overall mean specific performance loss rate, σ m The overall specific property loss rate includes all specific property loss rates of the base metal, weld, and heat-affected zone in nitrogen and at different hydrogen doping ratios. Standardization is performed only within the same performance range.

[0059] S4. Calculate the hydrogen embrittlement sensitivity evaluation index I based on the standardized specific performance loss rate PC ;

[0060] The performance loss rate matrix B has dimensions a × b, where a is the number of samples, i.e., the total number of conditions with different hydrogen doping environments and weld joint area combinations, and b is the number of performance loss rate types. Each row in B represents the performance loss rate for different performance indicators under the same condition, and each column represents the performance loss rate for different conditions with the same performance indicator. The covariance matrix Cov(B) is calculated based on the performance loss rate matrix B. Its dimensions are b × b. The formula for calculating the covariance matrix Cov(B) is as follows:

[0061]

[0062] Among them, B T is the transposed matrix of the performance loss rate matrix B, whose dimension is b×a;

[0063] The eigenvalue λ is calculated according to the equation det(Cov(B)-λI)=0 i , i=1,2,...,b, where λ1>λ2>...>λ b , det() is the determinant calculation of the matrix, I is the identity matrix, and λ i Substitute the homogeneous linear equations (Cov(B)-λ i I) v=0 to obtain the feature vector v i , and using the formula For the eigenvector v i Normalization is performed to obtain the normalized eigenvector u i ,in

[0064] According to I PCi =Bu i Obtain the hydrogen embrittlement sensitivity index I of different principal component directions PCi ;

[0065] according to Get the variance explanation rate β i , take β i >70% corresponding to I PCi As the final hydrogen embrittlement sensitivity evaluation index I PC .

[0066] S5. Perform scanning electron microscopy analysis on the slow strain rate tensile fracture to determine the risk level of hydrogen-induced cracking of the sample;

[0067] Cleavage fracture mode: A brittle fracture along a specific crystallographic plane, with a tongue-shaped, river-shaped, or cleavage-step fracture surface, and no obvious plastic deformation. Ductile fracture mode: A fracture with significant plastic deformation, characterized by dimples formed by microporous aggregation and shear tearing. Quasi-cleavage fracture mode: A hybrid mode with both cleavage and ductile fracture characteristics, with a fracture surface showing localized facets and tear ridges, accompanied by a small number of dimples and secondary cracks.

[0068] Ten areas are observed at the center of each fracture surface at a magnification of 1000 times. If the number of areas corresponding to the cleavage fracture mode is greater than 5, the hydrogen-induced cracking level of the sample is considered to be high risk; if the number of areas corresponding to the ductile fracture mode is greater than 5, the hydrogen-induced cracking level of the sample is considered to be low risk; if the number of areas corresponding to the quasi-cleavage fracture mode is greater than 5 or the number of areas corresponding to the cleavage fracture mode, the ductile fracture mode, and the quasi-cleavage fracture mode are all less than 5, that is, the fracture surface of the sample is mainly characterized by the quasi-cleavage fracture mode, and the hydrogen-induced cracking level of the sample is considered to be medium risk.

[0069] S6. Establishment I PC Relationship with hydrogen-induced cracking risk level, determine the I PC scope.

[0070] in I PC When n2 is less than I PC <n3 corresponds to a medium risk of hydrogen induced cracking, and when n4<I PC The corresponding hydrogen-induced cracking level is high risk, where n1≤n2≤n3≤n4, n1 and n2 represent all I PC After the values ​​are sorted from large to small, the hydrogen-induced cracking risk level is at the junction of low risk and medium risk, where n1 corresponds to the sample hydrogen-induced cracking risk level of low risk, n2 corresponds to the sample hydrogen-induced cracking risk level of medium risk, n3 and n4 represent all I PC After the values ​​are sorted from large to small, the hydrogen-induced cracking risk level is at the junction of medium risk and high risk, where n3 corresponds to a medium risk hydrogen-induced cracking risk level for the sample, and n4 corresponds to a high risk hydrogen-induced cracking risk level for the sample.

[0071] Example 2

[0072] The X65 pipeline steel welded joint was used as the test object, and slow strain rate tensile and fracture toughness tests were carried out in five different environments. The sampling diagram is shown in the figure. Figure 1 、 Figure 2 As shown in Figure 1, BM represents the base material area, WM represents the weld area, HAZ represents the heat affected zone area, and the atmosphere is pure nitrogen and a mixed atmosphere with hydrogen and nitrogen with a hydrogen doping ratio of 5%, 10%, 15%, and 20%, respectively. The nitrogen environment is used as an inert gas environment. The specific property loss rate is calculated based on the results of slow strain rate tensile and fracture toughness tests in five different environments, as shown in Table 1.

[0073] Table 1 Specific performance loss rates

[0074]

[0075] According to the method of the present invention, different eigenvalues ​​λ are calculated i The corresponding eigenvector u i , and calculate the variance explanation rate β i , where β1 = 78.7%, proving that it retains most of the effective information, and its eigenvalue u1 = [0.48, 0.42, 0.57, 0.52] T In addition, β2 = 20.5%, β3 = 0.8%, and β4 is approximately 0, so the principal component direction corresponding to β1 is selected to represent the hydrogen embrittlement sensitivity evaluation index I PC , as shown in Table 2.

[0076] Table 2 The principal component direction corresponding to β1 represents the hydrogen embrittlement sensitivity evaluation index I PC

[0077]

[0078] Among them I PC Negative values ​​appear (for example, when the base material has a hydrogen doping ratio of 10%, I PC is -1.78), which essentially reflects that the hydrogen embrittlement sensitivity under this condition is lower than the overall average level. The relative quantitative characteristics enable data from different regions and different hydrogen concentrations to be uniformly mapped to the same evaluation system.

[0079] According to the method of the present invention, the fracture surfaces of the samples under different gas environments and corresponding area combinations of the weld joints are observed and the corresponding hydrogen-induced cracking risk levels are determined. PC Relationship with hydrogen-induced cracking risk level, such as Figure 3 As shown in the figure, I PC It has a good correlation with the hydrogen-induced cracking risk level, proving that the hydrogen embrittlement sensitivity index obtained by combining multiple performance indicators can truly reflect the actual hydrogen embrittlement sensitivity of the material. PC <0.92, it is a low risk area for hydrogen-induced cracking. PC <4.25, it is the medium risk area of ​​hydrogen-induced cracking. PC When the value is >6.48, it is a high-risk area for hydrogen-induced cracking.

[0080] Therefore, the present invention adopts the above-mentioned test method for evaluating the hydrogen embrittlement sensitivity index of welded joints using the principal component analysis method. The principal component analysis method is used to innovatively integrate the multi-dimensional performance loss rate in a hydrogen environment into a one-dimensional comprehensive indicator, while retaining the main information, eliminating the redundancy and conflict between the indicators, and establishing a relationship between the risk degree of hydrogen-induced cracking and the performance indicators. It can improve the statistical significance of small sample data and obtain a more objective and comprehensive hydrogen embrittlement sensitivity evaluation system.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.​

Claims

1. A test method for evaluating the hydrogen embrittlement sensitivity index of welded joints using principal component analysis, characterized in that: The following steps are involved: S1. Process the pipe weld joint into a notched round bar slow strain rate tensile specimen. Notches are processed in the base metal, weld, and heat-affected zone. Slow strain rate tensile tests are then performed in a hydrogen environment and an inert gas environment. S2. Process the pipeline weld joint into a fracture toughness step-notch CT specimen, process notches in the base material, weld, and heat-affected zone, and conduct fracture toughness tests in hydrogen and inert gas environments respectively; S3. Calculate the specific performance loss rate under the hydrogen-doped environment based on the data obtained in S1 and S2, and perform standardization on the specific performance loss rate; S4. Calculate the hydrogen embrittlement sensitivity evaluation index I based on the standardized specific performance loss rate PC ; S5. Perform scanning electron microscopy analysis on the slow strain rate tensile fracture to determine the risk level of hydrogen-induced cracking of the sample; S6. Establishment I PC Relationship with hydrogen-induced cracking risk level, determine the I PC scope.

2. The method for evaluating the hydrogen embrittlement sensitivity index of a welded joint using principal component analysis according to claim 1, wherein: In S1, samples of the base material, weld and heat-affected zone machined notches were taken at 12, 3 and 6 o'clock positions of the pipeline welding to conduct repeated tests, and the notch tensile strength NTS, the maximum displacement l1 of the extensometer and the cross-sectional area A1 of the fracture were obtained; The calculation formula of elongation δ is as follows: Where l0 is the original gauge length, l1 is the maximum displacement of the extensometer during the stretching process; The calculation formula of the cross-sectional reduction rate ψ is as follows: Among them, A0 is the original cross-sectional area of ​​the test section, and A1 is the cross-sectional area of ​​the fractured part of the test section.

3. The method for evaluating the hydrogen embrittlement sensitivity index of a welded joint using principal component analysis according to claim 2, wherein: S2 obtains the fracture toughness value J of the weld joint.

4. The method for evaluating the hydrogen embrittlement sensitivity index of welded joints using principal component analysis according to claim 3, wherein: Specific performance loss rate I in hydrogen-doped S3 environment m The calculation formula is as follows: Among them, m0 is the specific performance of the sample in an inert gas environment, m H is the specific performance of the sample in a hydrogen-doped environment; when m is δ, Im is the elongation loss rate I δ ; When m is ψ, I m is the area reduction loss rate I ψ ; When m is NTS, I m is the notch tensile strength loss rate I NTS ; When m is J, I m is the fracture toughness loss rate I J ; Specific performance loss rate I under hydrogen doping environment m The calculation formula for normalization is as follows: Among them, Z m is the normalized specific performance loss rate, is the overall mean specific performance loss rate, σ m is the standard deviation of the overall specific performance loss rate.

5. The method for evaluating the hydrogen embrittlement sensitivity index of welded joints using principal component analysis according to claim 4, wherein: The dimension of the performance loss rate matrix B in S4 is a×b, where a is the number of samples, that is, the total number of conditions with different hydrogen doping environments and weld joint area combinations, and b is the number of performance loss rate types. The covariance matrix Cov(B) is calculated based on the performance loss rate matrix B, and its dimension is b×b. The calculation formula of the covariance matrix Cov(B) is as follows: Among them, B T is the transposed matrix of the performance loss rate matrix B, whose dimension is b×a; According to the equation det(Cov(B)-λI)=0, the eigenvalues ​​λi, i=1,2,…,b, where λ1>λ2>…>λ b , det() is the determinant calculation of the matrix, I is the identity matrix, and λ i Substitute the homogeneous linear equations (Cov(B)-λ i I) v=0 to obtain the feature vector v i , and using the formula For the eigenvector v i Normalization is performed to obtain the normalized eigenvector u i ,in According to I PCi =Bu i Obtain the hydrogen embrittlement sensitivity index I of different principal component directions PCi ; according to Get the variance explanation rate β i , take β i >70% corresponding to I PCi As the final hydrogen embrittlement sensitivity evaluation index I PC .

6. The method for evaluating the hydrogen embrittlement sensitivity index of welded joints using principal component analysis according to claim 5, characterized in that: In S5, 10 areas are observed at the center of each fracture at a magnification of 1000 times. If the number of areas corresponding to the cleavage fracture mode is greater than 5, the hydrogen-induced cracking level of the sample is considered to be high risk; if the number of areas corresponding to the ductile fracture mode is greater than 5, the hydrogen-induced cracking level of the sample is considered to be low risk; if the number of areas corresponding to the quasi-cleavage fracture mode is greater than 5 or the number of areas corresponding to the cleavage fracture mode, the ductile fracture mode, and the quasi-cleavage fracture mode are all less than 5, that is, the sample fracture is mainly characterized by the quasi-cleavage fracture mode, and the hydrogen-induced cracking level of the sample is considered to be medium risk.

7. The method for evaluating the hydrogen embrittlement sensitivity index of welded joints using principal component analysis according to claim 6, wherein: In S6 PC When n2 is less than I PC <n3 corresponds to a medium risk of hydrogen induced cracking, and when n4<I PC The corresponding hydrogen-induced cracking level is high risk, where n1≤n2≤n3≤n4, n1 and n2 represent all I PC After the values ​​are sorted from large to small, the hydrogen-induced cracking risk level is at the junction of low risk and medium risk, where n1 corresponds to the sample hydrogen-induced cracking risk level of low risk, n2 corresponds to the sample hydrogen-induced cracking risk level of medium risk, n3 and n4 represent all I PC After the values ​​are sorted from large to small, the hydrogen-induced cracking risk level is at the junction of medium risk and high risk, where n3 corresponds to a medium risk hydrogen-induced cracking risk level for the sample, and n4 corresponds to a high risk hydrogen-induced cracking risk level for the sample.

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