Experimental research method for weld joint micro-damage induced crack propagation under large-span temperature difference

Abstract

The invention discloses an experimental research method for weld joint micro-damage induced crack propagation under a large-span temperature difference, and belongs to the technical field of oil and gas storage and transportation. Comprising the following steps: 1, inducing a welding part to generate micro-damage by utilizing an experiment table, so that a crack appears on a welding seam of the workpiece, and changing the hydrogen content, welding parameters and the performance of a welding material so as to induce the welding seam crack under different conditions; step 2, testing the weld crack obtained in the step 1 by using an electron probe analyzer, a transmission electron microscope (TEM), an X-ray diffraction analyzer (XRD) and a field emission scanning electron microscope (FESEM) so as to analyze an induced stress field near the tip of the weld crack, determining the influence of the weld crack tip microscopic accumulated damage on the weld crack propagation path and propagation mode; and step 3, based on a J integral theoretical equation and Gaussian integral, establishing an integral mode of the upper and lower surfaces of the weld material crack obtained in the step 1, and based on the J integral and an equivalent integral region method, carrying out secondary development on finite element software.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas storage and transportation, and in particular to an experimental research method for micro-damage induced crack propagation of welds under large-span temperature difference. Background Art

[0002] For high-pressure vessels serving in sour gas fields or gas fields in areas with large temperature differences between day and night, they will be affected by factors such as hydrogen permeation, thermal expansion and contraction, and fatigue. Combined with the theory of cumulative damage and progressive material damage, the above factors can easily cause micro-damage to the welds of such high-pressure vessels and induce crack propagation, thus posing a safety hazard to the efficient and safe development of such gas fields.

[0003] Based on the "fitness for use" and "weakest link" principles, accurately determining whether in-service pressure vessels with weld crack defects can continue to be used safely under specified operating conditions is an essential requirement for ensuring the safe operation of pressure vessels and is also an engineering need. If the assessment results are too conservative, frequent repairs and replacements will occur, which will not only affect the normal production of the gas field but also cause economic losses; if the assessment results are too bold, allowing them to exist will leave safety hazards. Pressure vessels containing sour gases have higher manufacturing costs, which puts higher demands on maximizing economic development. Therefore, how to improve the safety assessment accuracy of high-pressure vessels in the above environment has always been a technical bottleneck that has plagued the efficient and safe development of such gas fields.

[0004] Micro-damage is micron-to-nano-scale damage in the structure, including crystal defects, material performance degradation, and micron-scale cracks. From initiation to failure, cracks induced by micro-damage go through three stages: crack initiation, steady-state expansion, and unstable expansion. When the stress intensity factor K at the crack tip is greater than KIC (fracture criterion), the crack expands, leading to fracture failure. There are fruitful research results on the safety hazards faced by high-pressure vessels with crack defects. However, the existing research on the dynamic mechanism of micro-damage-induced crack expansion is basically proposed for a certain type of metal material or a certain deformation stage, which cannot be universally applied. This paper proposes an experimental research method for micro-damage-induced crack expansion in welds under large-span temperature difference, aiming to obtain the degradation law of mechanical properties and the change of fracture form under micro-damage, as well as the quantitative characterization of micro-damage-induced crack expansion, so as to provide the microscopic dynamic mechanism of micro-damage-induced crack expansion in welds under large-span temperature difference for the subsequent crack expansion and residual strength evaluation of high-pressure vessel welds. Summary of the invention

[0005] The purpose of the present invention is to provide an experimental research method for micro-damage induced crack propagation in welds under large-span temperature difference, so as to solve the problems raised in the above-mentioned background technology.

[0006] In view of the above problems, the technical solution proposed by the present invention is: An experimental research method for micro-damage induced crack propagation in welds under large-span temperature difference comprises the following steps: Step 1: Use the test bench to induce micro-damage to the welded parts, so that cracks appear in the weld of the workpiece, and change the hydrogen content at the weld, welding parameters, and properties of welding materials to induce weld cracks under different conditions; Step 2: Use electron probe analyzer, transmission electron microscope (TEM), X-ray diffraction analyzer (XRD) and field emission scanning electron microscope (FESEM) to test the weld crack obtained in step 1 to analyze the induced stress field near the weld crack tip and clarify the influence of microscopic accumulated damage at the weld crack tip on the weld crack propagation path and propagation mode; Step 3: Based on the theoretical equation of J integral and Gauss integral, the integral mode of the upper and lower surfaces of the crack of the weld material obtained in step 1 is established. Based on J integral and equivalent integral area method, the finite element software is secondary developed, and Python code is written to solve KⅠ and KⅡ under different girths to realize the calculation of stress intensity factor under non-uniform load and irregular crack morphology; Step 4: Analyze the influence of the operating conditions of the welded parts, the defect location of the crack, and the size parameters of the crack on the stress intensity factor, and determine the microscopic dynamic mechanism of crack propagation induced by micro-damage of the weld under large-span temperature difference.

[0007] As a preferred technical solution of the present invention, the large-span temperature difference working condition simulation test bench in step one includes a fatigue testing machine, the working chamber of the fatigue pressure testing machine is divided into a low temperature zone and a high temperature zone, the outer cover of the fatigue pressure testing machine is provided with a nitrogen protective shell, and the nitrogen protective shell is connected to a nitrogen discharge port, the large-span temperature difference working condition simulation test bench includes a fatigue testing machine and also includes an acid gas tank, a constant temperature and pressure gas buffer tank, a pair of reheaters, a liquid nitrogen tank, and a vaporizer, liquid nitrogen is stored in the liquid nitrogen tank, the liquid nitrogen tank is connected to the vaporizer, one of the reheaters is connected to the vaporizer, and at the same time is connected to the nitrogen protective shell through a regulating valve 1, and is connected to the constant temperature and pressure gas buffer tank through a regulating valve 2, the liquid nitrogen tank is connected to the constant temperature and pressure gas buffer tank through a regulating valve 3, and the constant temperature and pressure gas buffer tank is connected to the low temperature zone through a regulating valve 5.

[0008] As a preferred technical solution of the present invention, acid gas is stored in the acid gas tank, another reheater is connected to the acid gas tank, and is connected to the constant temperature and pressure gas buffer tank through a regulating valve 4, and the constant temperature and pressure gas buffer tank is connected to the high temperature zone through a regulating valve 6.

[0009] Specifically, a solenoid valve is installed at the nitrogen discharge port to control the discharge of nitrogen in the nitrogen protection shell. A temperature control loop is set between the constant temperature and pressure gas buffer tank and regulating valve 2, regulating valve 3, and regulating valve 4 to control the temperature in the constant temperature and pressure gas buffer tank, and a temperature control loop is also set between the nitrogen protection shell and regulating valve 1 to control the temperature in the nitrogen protection shell. The constant temperature and pressure gas buffer tank, the nitrogen protection shell, a pair of reheaters, a vaporizer, an acid gas tank, and a liquid nitrogen tank are all connected by pipelines, and regulating valve 1, regulating valve 2, regulating valve 3, regulating valve 4, regulating valve 5, and regulating valve 6 are all installed on the pipeline.

[0010] As a preferred technical solution of the present invention, the transmission electron microscope (TEM) in step 2 is used to test and analyze the microstructure of the micro-area before and after micro-damage of the weld; the X-ray diffraction analyzer (XRD) is used to analyze the phase composition and relative content of the micro-damaged part of the weld; the field emission scanning electron microscope (FESEM) is used to test the microstructure and morphology of the micro-damaged part of the weld, and characterize the number, morphology, size, and spatial arrangement of the grains and crystal defects in the micro-damaged part of the weld.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows: the experimental research method of micro-damage induced crack propagation of welds under large-span temperature difference simulates and analyzes the microstructure damage propagation characteristics of the weld crack tip under large-span alternating temperature difference under different macroscopic parameters such as alternating temperature difference, service time, hydrogen content, structural size and material properties, explores the influence of macroscopic parameters on its damage propagation, and tests the microstructure of the damaged part to reveal its micro-damage evolution law and mechanism; The experimental research method of micro-damage induced crack propagation in welds under large-span temperature difference simulates and analyzes the initial propagation path of three-dimensional crack propagation in welds under large-span alternating temperature difference under different macro-parameter conditions, explores the influence of macro-parameters on the crack tip angle, and tests the microstructure of the fracture site to determine its correlation with material heterogeneity. This experimental research method on micro-damage-induced crack propagation in welds under large-span temperature difference constructs a quantitative relationship between the microscopic accumulated damage at the crack tip of the weld under large-span alternating temperature difference, the quantitative characteristics of the micro-region dimples, and the elastic-plastic mechanical behavior of the microstructure at the crack tip of the weld, and proposes a refined description method for the heterogeneity of weld materials and the correlation between their microstructure and material properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 A method block diagram of an experimental research method for micro-damage induced crack propagation in welds under large-span temperature difference disclosed in an embodiment of the present invention; Figure 2 A schematic diagram of the structure of an experimental platform for an experimental research method for micro-damage-induced crack propagation in welds under large-span temperature difference disclosed in an embodiment of the present invention; Figure 3 A block diagram of a finite element numerical simulation method for an experimental research method of micro-damage induced crack propagation in a weld under a large-span temperature difference disclosed in an embodiment of the present invention; Figure 4 The invention discloses a numerical simulation calculation process of stress intensity factor under non-uniform load and irregular crack morphology in the experimental research method of micro-damage induced crack propagation of weld under large-span temperature difference disclosed in the embodiment of the invention. DETAILED DESCRIPTION

[0013] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0014] See also Figure 1 - Figure 4 The present invention provides a technical solution: an experimental research method for micro-damage induced crack propagation of welds under large-span temperature difference, comprising the following steps: Step 1: Use the test bench to induce micro-damage to the welded parts, so that cracks appear in the weld of the workpiece, and change the hydrogen content at the weld, welding parameters, and properties of welding materials to induce weld cracks under different conditions; Step 2: Use electron probe analyzer, transmission electron microscope (TEM), X-ray diffraction analyzer (XRD) and field emission scanning electron microscope (FESEM) to test the weld crack obtained in step 1 to analyze the induced stress field near the weld crack tip and clarify the influence of microscopic accumulated damage at the weld crack tip on the weld crack propagation path and propagation mode; Step 3: Based on the theoretical equation of J integral and Gauss integral, the integral mode of the upper and lower surfaces of the crack of the weld material obtained in step 1 is established. Based on J integral and equivalent integral area method, the finite element software is secondary developed, and Python code is written to solve KⅠ and KⅡ under different girths to realize the calculation of stress intensity factor under non-uniform load and irregular crack morphology; Step 4: Analyze the influence of the operating conditions of the welded parts, the defect location of the crack, and the size parameters of the crack on the stress intensity factor, and determine the microscopic dynamic mechanism of crack propagation induced by micro-damage of the weld under large-span temperature difference.

[0015] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0016] See also Figure 1 - Figure 4 The present invention provides a technical solution: an experimental research method for micro-damage induced crack propagation of welds under large-span temperature difference. The large-span temperature difference working condition simulation test bench in step one includes a fatigue testing machine, the working chamber of the fatigue pressure testing machine is divided into a low temperature zone and a high temperature zone, the outer cover of the fatigue pressure testing machine is provided with a nitrogen protective shell, and the nitrogen protective shell is connected with a nitrogen discharge port. The large-span temperature difference working condition simulation test bench includes a fatigue testing machine and also includes an acid gas tank, a constant temperature and pressure gas buffer tank, a pair of reheaters, a liquid nitrogen tank, and a vaporizer. Liquid nitrogen is stored in the liquid nitrogen tank, and the liquid nitrogen tank is connected to the vaporizer, one of the reheaters is connected to the vaporizer, and at the same time, it is connected to the nitrogen protective shell through a regulating valve 1 and connected to the constant temperature and pressure gas buffer tank through a regulating valve 2. The liquid nitrogen tank is connected to the constant temperature and pressure gas buffer tank through a regulating valve 3, and the constant temperature and pressure gas buffer tank is connected to the low temperature zone through a regulating valve 5.

[0017] In one embodiment of the present invention, further, acid gas is stored in the acid gas tank, another reheater is connected to the acid gas tank, and is connected to the constant temperature and pressure gas buffer tank through a regulating valve 4, and the constant temperature and pressure gas buffer tank is connected to the high temperature zone through a regulating valve 6.

[0018] Effects In one embodiment of the present invention, further, the acid gas in the acid gas tank enters the gas mixer, and the gas in the natural gas high-pressure cylinder enters the gas mixer through the regulating valve. According to a certain ratio, the two gases are mixed in the gas mixer (maintaining a certain pressure), and the mixed gas medium enters one of the reheaters to be heated to high-temperature gas (temperature is about 60°C). After circulation, the high-temperature gas supplies gas to the constant temperature and constant pressure mixed gas buffer tank through the regulating valve 4. In order to adjust the temperature and pressure in the constant temperature and constant pressure mixed gas buffer tank to be constant (the constant temperature range is determined according to the design parameters of the heat exchanger, about 55°C to 60°C), the high-temperature mixed gas heated by one of the reheaters is mixed with the low-temperature nitrogen through the regulating valve 3 through the regulating valve 2 to achieve the temperature and pressure regulation control in the constant temperature and constant pressure gas buffer tank. The micro-damage experiment is carried out in a high temperature and toxic gas environment. In order to avoid gas leakage from various components of the test bench during the micro-damage experiment and ensure the personal safety of the experimenters, the test bench is isolated by a nitrogen protective shell. The nitrogen entering the nitrogen protective shell is adjusted by the regulating valve 1 so that the nitrogen fills the entire enclosed space. High-temperature or low-temperature mixed gas is supplied to the working chamber through regulating valves 5 and 6. High-temperature mixed gas circulation is achieved through regulating valves 2 and 4. A part of the acidic mixed gas enters another reheater to be heated as high-temperature mixed gas, and the other part of the acidic mixed gas is supplied to the constant temperature and pressure mixed gas buffer tank through regulating valve 2. In order to achieve gas circulation, the test bench fine-tunes the mixed gas pressure by adjusting the temperature. When gas circulation is required, increase the temperature of a pair of reheaters, increase the pressure, open regulating valves 2 and 4, and close regulating valves 5 and 6 to allow the mixed gas to enter the constant temperature and pressure mixed gas buffer tank to achieve self-circulation.

Claims

1. An experimental research method for micro-damage induced crack propagation in welds under large-span temperature difference, characterized in that: The following steps are involved: Step 1: Use the test bench to induce micro-damage to the welded parts, so that cracks appear in the weld of the workpiece, and change the hydrogen content at the weld, welding parameters, and properties of welding materials to induce weld cracks under different conditions; Step 2: Use electron probe analyzer, transmission electron microscope (TEM), X-ray diffraction analyzer (XRD) and field emission scanning electron microscope (FESEM) to test the weld crack obtained in step 1 to analyze the induced stress field near the weld crack tip and clarify the influence of microscopic accumulated damage at the weld crack tip on the weld crack propagation path and propagation mode; Step 3: Based on the theoretical equation of J integral and Gauss integral, the integral mode of the upper and lower surfaces of the crack of the weld material obtained in step 1 is established. Based on J integral and equivalent integral area method, the finite element software is secondary developed, and Python code is written to solve KⅠ and KⅡ under different girths to realize the calculation of stress intensity factor under non-uniform load and irregular crack morphology; Step 4: Analyze the influence of the operating conditions of the welded parts, the defect location of the crack, and the size parameters of the crack on the stress intensity factor, and determine the microscopic dynamic mechanism of crack propagation induced by micro-damage of the weld under large-span temperature difference.

2. The experimental research method for micro-damage induced crack propagation of welds under large-span temperature difference according to claim 1 is characterized in that: The large-span temperature difference working condition simulation test bench in step one includes a fatigue testing machine, the working chamber of the fatigue pressure testing machine is divided into a low temperature zone and a high temperature zone, the outer cover of the fatigue pressure testing machine is provided with a nitrogen protective shell, the nitrogen protective shell is connected with a nitrogen discharge port, the large-span temperature difference working condition simulation test bench includes a fatigue testing machine and also includes an acid gas tank, a constant temperature and pressure gas buffer tank, a pair of reheaters, a liquid nitrogen tank, and a vaporizer, liquid nitrogen is stored in the liquid nitrogen tank, the liquid nitrogen tank is connected to the vaporizer, one of the reheaters is connected to the vaporizer, and at the same time, it is connected to the nitrogen protective shell through a regulating valve 1, and is connected to the constant temperature and pressure gas buffer tank through a regulating valve 2, the liquid nitrogen tank is connected to the constant temperature and pressure gas buffer tank through a regulating valve 3, and the constant temperature and pressure gas buffer tank is connected to the low temperature zone through a regulating valve 5.

3. The experimental research method for micro-damage induced crack propagation of welds under large-span temperature difference according to claim 2 is characterized in that: The acid gas tank stores acid gas, the other reheater is connected to the acid gas tank, and is connected to the constant temperature and pressure gas buffer tank through a regulating valve 4, and the constant temperature and pressure gas buffer tank is connected to the high temperature zone through a regulating valve 6.

4. The experimental research method for micro-damage induced crack propagation of welds under large-span temperature difference according to claim 1 is characterized in that: The transmission electron microscope (TEM) in the step 2 is used to test and analyze the microstructure of the micro-area before and after micro-damage of the weld; the X-ray diffraction analyzer (XRD) is used to analyze the phase composition and relative content of the micro-damaged part of the weld; the field emission scanning electron microscope (FESEM) is used to test the microstructure and morphology of the micro-damaged part of the weld, and characterize the number, morphology, size, and spatial arrangement of the grains and crystal defects in the micro-damaged part of the weld.

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