Method for evaluating hydrogen embrittlement cracking resistance of a backing weld of a pipe girth weld joint

Abstract

The present application belongs to the field of hydrogen transmission pipeline safety evaluation, and provides a method for evaluating the hydrogen-induced brittle cracking resistance of a backing weld of a pipe girth joint. A single-edge notched specimen of the pipe girth joint is installed on a testing machine, and the specimen is placed in a temperature-controlled high-pressure environment box. A pressure head applies a load to the specimen at a slow displacement growth rate until the specimen breaks, and a load-displacement curve is recorded. The maximum load on the load-displacement curve is taken as the load at which a crack is generated, and the envelope area of the load-displacement curve before the maximum load is reached is taken as the crack formation work. The envelope area of the load-displacement curve after the maximum load is exceeded is taken as the crack propagation work, and the resistance of the material to cracking and crack propagation under the corresponding environment is evaluated. The sensitivity of the material to hydrogen-induced brittle cracking is evaluated according to the fracture morphology, and fibrous cracking is ductile cracking, and crystalline cracking is brittle cracking. The present application solves the problem that the backing weld is difficult to evaluate in terms of safety according to the existing material hydrogen environment compatibility test method due to the small thickness.

Description

Technical Field

[0001] This invention belongs to the technical field of hydrogen pipeline safety evaluation, and provides a method for evaluating the ability of the root pass of a pipe ring weld joint to resist hydrogen-induced embrittlement cracking. It is used to evaluate the ability of the root pass of a metal pipe ring weld joint to resist hydrogen-induced embrittlement cracking under service environment. Background Technology

[0002] my country's hydrogen energy industry is developing rapidly, with an increasing need to transport hydrogen from production sites to end users. Pipeline transportation is a crucial method for large-scale, long-distance hydrogen transport. Utilizing existing natural gas pipelines for hydrogen or hydrogen-blended transport is the most economical approach, but a suitability assessment for hydrogen transport must be conducted on the natural gas pipelines before conversion. Existing long-distance natural gas pipelines are primarily constructed using ordinary pipeline steel, which is susceptible to hydrogen-induced cracking in pure or blended hydrogen environments, threatening the safety of the steel pipelines.

[0003] During pipeline hydrogen transport, the hydrogen concentration distribution within the pipe body is not uniform, gradually decreasing from the inner wall to the outer wall (ignoring stress field effects). Therefore, the hydrogen concentration in the root pass of the circumferential weld joint on the hydrogen-facing side is high. Simultaneously, the root pass of the circumferential weld joint is prone to geometrical abrupt changes such as incomplete penetration and poor forming, leading to stress concentration at the root pass location during pipeline service. Hydrogen within the pipe body also diffuses and accumulates towards the high-stress area under the influence of stress gradients. Driven by both concentration and stress gradient diffusion, the actual hydrogen concentration in the root pass is highly likely to reach the critical concentration for hydrogen-induced cracking, thus posing a high risk of hydrogen-induced cracking. Furthermore, the welding materials and welding processes used for the root pass in pipeline circumferential welding are often different from those used for filler and capping passes, resulting in different microstructures and properties. Clearly, the crack resistance of the root pass must be assessed when evaluating the suitability of the pipeline for hydrogen transport.

[0004] Evaluating the hydrogen environment compatibility of materials is a crucial step in assessing the suitability of existing pipelines for hydrogen transportation. Currently, various standard experimental methods have been established, including environmental slow tensile testing, fatigue life testing, fracture toughness testing, and crack propagation rate testing. However, these methods unfortunately cannot be used to assess the hydrogen embrittlement risk of the root pass of circumferential weld joints. This is because the thickness of the root pass is generally no more than 2 mm. When machining the round bar specimens used for slow tensile or fatigue tests, the root pass is almost entirely removed. Furthermore, specimens tested for fracture toughness or crack propagation rate require notches and pre-existing cracks. The combined depth of the notch and the initial depth of the pre-existing crack already exceeds the thickness range of the root pass. Therefore, an alternative approach must be explored to investigate the resistance of the root pass to hydrogen-induced cracking under the intended service hydrogen environment.

[0005] If a method could be developed to quantitatively evaluate the resistance of pipeline circumferential weld joints to crack initiation and propagation under a predetermined hydrogen environment, and to determine whether hydrogen-induced embrittlement has occurred, it would have significant practical implications for assessing the suitability of existing natural gas pipelines for hydrogen transportation. Summary of the Invention

[0006] This invention addresses the limitation of standard methods for testing material compatibility in hydrogen environments in assessing the hydrogen embrittlement sensitivity of the root pass of pipe ring welds. It provides a method for evaluating the ability of the root pass of a metal pipe ring weld to resist hydrogen-induced embrittlement cracking under service conditions. The technical solution adopted in this invention is as follows:

[0007] A method for evaluating the resistance of the root pass of a pipe circumferential weld joint to hydrogen-induced embrittlement includes the following steps:

[0008] (1) Install the single-sided notch specimen of the pipe ring weld joint on a testing machine with three-point bending test function, and place the specimen in a temperature-controlled high-pressure environment chamber.

[0009] (2) After removing the air from the temperature-controlled high-pressure environment chamber in step (1), introduce pure hydrogen or hydrogen-containing gas at a predetermined pressure.

[0010] (3) The indenter applies a load to the specimen at a slow displacement growth rate until the specimen breaks, and the load-displacement curve is recorded during the process.

[0011] (4) The maximum load on the load-displacement curve P max As the load that generates cracks, to achieve the maximum load. P max The previous load-displacement curve envelope area was successfully used as a crack shape. W f To exceed the maximum load P max The area of ​​the enveloping load-displacement curve after crack propagation is used as the work done to crack propagation. W d , W f and W d These are used to evaluate the material's resistance to crack initiation and crack propagation under corresponding environments;

[0012] (5) After the test is completed, the temperature of the temperature-controlled high-pressure environment chamber is reduced to room temperature and the test gas is discharged to one atmosphere. Then, the test gas is replaced with nitrogen and the temperature-controlled high-pressure environment chamber is opened and the sample is taken out.

[0013] (6) Further assess the sensitivity of the material to hydrogen-induced embrittlement based on the fracture morphology. Fibrous fractures are ductile fractures, while crystalline fractures are embrittlement fractures.

[0014] Preferably, the sample in step (1) is a single-sided notch sample, with the notch length along the weld length direction, the notch depth along the pipe wall thickness direction, the notch opening on the root pass weld, the notch cross-sectional shape being V-shaped, the notch depth being 0.5 mm, and the stress concentration factor being greater than or equal to 3.

[0015] Preferably, the testing machine in step (1) has the following functions: the function of conducting a three-point bending test under high pressure gas environment, the function of loading at a slow displacement growth rate, and the function of recording the load-displacement curve during the bending test.

[0016] Preferably, the temperature-controlled high-pressure environment chamber is made of 304 or 316 stainless steel and can simulate the temperature and pressure environment of pure hydrogen or hydrogen-doped pipelines.

[0017] Preferably, the slow displacement growth rate in step (2) is ≤5×10 -5 mm / s.

[0018] Preferably, step (2) of replacing the air in the temperature-controlled high-pressure environment chamber and then introducing the test gas includes: first replacing the air with high-purity nitrogen three times, then replacing the high-purity nitrogen with the test gas three times, and finally introducing the test gas and pressurizing it to the pipeline design pressure or the maximum service pressure, with a pressurization rate of less than 1 MPa / min.

[0019] Preferably, the ability of the root pass of the pipe ring weld joint to resist cracking under a predetermined hydrogen environment and the possibility of hydrogen-induced embrittlement are evaluated by combining crack formation success, crack propagation energy and fracture analysis during bending.

[0020] The beneficial effects of this invention are:

[0021] Using the above method, the ability of the root pass of the pipe ring weld joint to resist hydrogen-induced embrittlement cracking under the action of hydrogen environment, notch and stress coupling in service can be evaluated, which solves the problem that the root pass is difficult to conduct safety assessment based on existing material hydrogen environment compatibility testing methods due to its small thickness. Attached Figure Description

[0022] Figure 1 This is a schematic diagram showing the position of the root pass weld in the pipe ring weld joint according to an embodiment of the present invention. (a) shows the position of the weld in the pipe, and (b) shows the position of the root pass weld in the pipe ring weld joint.

[0023] Figure 2 This is a schematic diagram of the load-displacement curve during the bending process of the root pass weld in an embodiment of the present invention. Detailed Implementation

[0024] like Figure 1As shown, the circumferential weld of the pipeline forms a circumferential weld joint, which includes a root pass 1 located on the inner wall of the pipeline, followed by filler passes 2, 3, 4, and 5 from the inside out, and a cap pass 6 located on the outer wall of the pipeline. The thickness of the root pass 1 generally does not exceed 2 mm. Existing testing equipment and methods cannot be directly used to study the resistance of the root pass to hydrogen-induced cracking in a predetermined service hydrogen environment.

[0025] The specific concept of this invention is as follows:

[0026] Data shows that the single-sided notched three-point bending test can be used to determine the fracture toughness of materials. The advantage of this method is that the root pass of the circumferential weld joint can be completely preserved during sample preparation. However, the problem is that the depth of the notch plus the pre-crack will still exceed the thickness of the root pass. Currently, there are no literature reports on using the three-point bending test to evaluate the compatibility of materials in a hydrogen environment, and the process parameters used in existing three-point bending tests in air environments are not suitable for evaluating the effects of hydrogen environments. This invention aims to evaluate the resistance to hydrogen-induced cracking of the root pass of a pipe circumferential weld joint by conducting a single-sided notched three-point bending test in a high-pressure hydrogen or hydrogen-containing gas environment. It can also assess the possibility of hydrogen-induced embrittlement cracking under simulated service hydrogen environments. Solutions are proposed for sample notch design, bending test process control, and the extraction of quantitative indicators to assess the crack resistance of the root pass.

[0027] The specific technical solutions of this invention are as follows:

[0028] A method for evaluating the resistance of the root pass of a pipe circumferential weld joint to hydrogen-induced embrittlement cracking includes the following steps:

[0029] (1) The designed pipe ring weld joint single-sided notch specimen is installed on a testing machine that can carry out three-point bending test, and the specimen is placed in a temperature-controlled high-pressure environment chamber;

[0030] The specimen was made from the actual pipe ring weld joint under study. The notch length is along the weld length direction, the notch depth is along the pipe wall thickness direction, the notch is made on the root pass weld, the notch cross-sectional shape is V-shaped, and the stress concentration factor is greater than or equal to 3. The recommended V-shaped notch dimensions are an opening angle of 40°, a depth of 0.5 mm, and a root radius of 0.1 mm.

[0031] The testing machine can perform three-point bending tests under high-pressure gas conditions, achieve slow displacement growth rate loading, and record load-displacement curves during the bending test. It is recommended that the displacement growth rate be controlled to not exceed 5 × 10⁻⁶. -5 mm / s.

[0032] The temperature-controlled high-pressure environment chamber is made of 304 or 316 stainless steel and can simulate the temperature and pressure environment of hydrogen-doped or hydrogen-transporting pipelines.

[0033] (2) After removing the air from the temperature-controlled high-pressure environment chamber in step (1), introduce pure hydrogen or hydrogen-containing gas at a predetermined pressure.

[0034] The gas emissions after the gas replacement, hydrogen filling, and loading tests shall be carried out in strict accordance with the safety specifications for high-pressure hydrogen-containing gas tests. The pressurization rate of hydrogen-containing gas introduced into the temperature-controlled high-pressure test chamber before loading begins shall be less than 1 MPa / min, and the gas emission rate after the loading test is completed shall also be less than 1 MPa / min.

[0035] The method of replacing the air with nitrogen and then replacing the nitrogen with the test gas before introducing hydrogen-containing gas into the temperature-controlled high-pressure environment chamber aims to control the impurity content in the test gas. The air removal step can also be carried out by vacuuming.

[0036] (3) The pressure head increases at a slow displacement rate (≤5×10) -5 A load (mm / s) is applied to the specimen until the specimen breaks, and the load-displacement curve is recorded during the process.

[0037] The load-displacement curve diagram is shown below. Figure 2 The maximum load on the curve is used P max The curve's envelope is divided into three parts: I, II, and III. The envelope areas of these three parts correspond to the elastic deformation work, respectively. W e Plastic deformation work W p and crack propagation function W d , W e Plus W p Collectively referred to as crack-shaped success W f . W d and W f The higher the value, the stronger the material's resistance to crack initiation and crack propagation in a hydrogen environment.

[0038] (4) The maximum load on the load-displacement curve P max As the load that generates cracks, to achieve the maximum load. P max The previous load-displacement curve envelope area was successfully used as a crack shape. W f To exceed the maximum load P maxThe area of ​​the enveloping load-displacement curve after crack propagation is used as the work done to crack propagation. W d , W f and W d These are used to evaluate the material's resistance to crack initiation and crack propagation under corresponding environments;

[0039] (5) After the test is completed, the temperature is lowered to room temperature and the test gas is discharged to one atmosphere. Then, the test gas is replaced with nitrogen and the temperature-controlled high-pressure test chamber is opened and the sample is taken out.

[0040] (6) Further assess the sensitivity of the material to hydrogen-induced embrittlement based on the fracture morphology. Fibrous fractures are ductile fractures, while crystalline fractures are embrittlement fractures.

[0041] The fracture surface of the bending fracture specimens after the test was analyzed by visual inspection and with the help of scanning electron microscopy and other technical means to determine whether brittle cracking occurred in the root pass of the circumferential weld joint of the pipeline in service.

[0042] Compared with the established methods for testing the hydrogen environment compatibility of materials, the beneficial effects of the above-mentioned technical solution in this invention are: it can quantitatively evaluate the hydrogen-induced crack initiation and propagation resistance of the root pass of the pipe ring weld joint, and can give the evaluation results of whether hydrogen-induced brittle cracking can occur under the combined action of material, hydrogen environment, and notch stress.

[0043] When implementing the above solution, any ordinary changes and substitutions made by those skilled in the art within the scope of this technical solution should be included within the protection scope of this invention.

Claims

1. A method for evaluating the resistance of the root pass of a pipe circumferential weld joint to hydrogen-induced embrittlement cracking, characterized in that, Includes the following steps: (1) Install the single-sided notch specimen of the pipe ring weld joint on a testing machine with three-point bending test function, and place the specimen in a temperature-controlled high-pressure environment chamber. (2) After removing the air from the temperature-controlled high-pressure environment chamber in step (1), introduce pure hydrogen or hydrogen-containing gas at a predetermined pressure. (3) The indenter applies a load to the specimen at a slow displacement growth rate until the specimen breaks, and the load-displacement curve is recorded during the process. (4) The maximum load on the load-displacement curve P max As the load that generates cracks, to achieve the maximum load. P max The previous load-displacement curve envelope area was successfully used as a crack shape. W f To exceed the maximum load P max The area of ​​the enveloping load-displacement curve after crack propagation is used as the work done to crack propagation. W d , W f and W d These are used to evaluate the material's resistance to crack initiation and crack propagation under corresponding environments; (5) After the test is completed, the temperature of the temperature-controlled high-pressure environment chamber is reduced to room temperature and the test gas is discharged to one atmosphere. Then, the test gas is replaced with nitrogen and the temperature-controlled high-pressure environment chamber is opened and the sample is taken out. (6) Further assess the sensitivity of the material to hydrogen-induced embrittlement based on the fracture morphology. Fibrous fractures are ductile fractures, while crystalline fractures are brittle fractures. The specimen in step (1) is a single-sided notch specimen. The notch length is along the weld length direction, the notch depth is along the pipe wall thickness direction, the notch is opened on the root pass weld, the notch cross-sectional shape is V-shaped, the notch depth is 0.5mm and the stress concentration factor is greater than or equal to 3.

2. The method for evaluating the resistance of the root pass of a pipe circumferential weld joint to hydrogen-induced embrittlement cracking according to claim 1, characterized in that, The testing machine in step (1) has the following functions: the function of conducting a three-point bending test under high pressure gas environment, the function of loading at a slow displacement growth rate, and the function of recording the load-displacement curve during the bending test.

3. The method for evaluating the resistance of the root pass of a pipe circumferential weld joint to hydrogen-induced embrittlement cracking according to claim 1, characterized in that, The temperature-controlled high-pressure environment chamber is made of 304 or 316 stainless steel and can simulate the temperature and pressure environment of pure hydrogen or hydrogen-doped pipelines.

4. The method for evaluating the resistance of the root pass of a pipe circumferential weld joint to hydrogen-induced embrittlement cracking according to claim 1, characterized in that, The slow displacement growth rate in step (2) is ≤5×10 -5 mm / s.

5. The method for evaluating the resistance of the root pass of a pipe circumferential weld joint to hydrogen-induced embrittlement cracking according to claim 1, characterized in that, Step (2) involves replacing the air in the temperature-controlled high-pressure environment chamber and then introducing the test gas. The steps include replacing the air with high-purity nitrogen three times, replacing the high-purity nitrogen with the test gas three times, and finally introducing the test gas and pressurizing it to the pipeline design pressure or the maximum service pressure. The pressurization rate is less than 1 MPa / min.

6. The method for evaluating the resistance of the root pass of a pipe circumferential weld joint to hydrogen-induced embrittlement cracking according to claim 1, characterized in that, By combining crack formation success, crack propagation energy, and fracture surface analysis during bending, the ability of the root pass of the pipe ring weld joint to resist cracking under a predetermined hydrogen environment and the possibility of hydrogen-induced embrittlement cracking are evaluated.

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