Research method for elastic-plastic mechanical behaviors of weld microstructure under large-span temperature difference

Abstract

The invention discloses an elastic-plastic mechanical behavior research method of a weld microstructure under a large-span temperature difference, and belongs to the technical field of oil and gas storage and transportation. Comprising the following steps: step 1, establishing a large-span temperature difference working condition simulation experiment table; and 2, based on the large-span temperature difference working condition simulation test bench in the step 1, performing a multi-scale mechanical experiment in combination with a mechanical tensile experiment under nanoindentation and different angles, and researching the evolution of a stress-strain curve of the weld microstructure under the large-span alternating temperature difference, so as to obtain the weld microstructure stress-strain curve of the weld microstructure under the large-span alternating temperature difference. Establishing a local elastic-plastic model of the weld microstructure under the large-span alternating temperature difference based on multi-factor coupling; and step 3, through establishment and solution of the local elastic-plastic model of the weld microstructure under the large-span alternating temperature difference obtained in the step 2 and the induced stress field collaborative kinetic model near the tip of the crack, analyzing the influence rule of the operation pressure, the temperature difference and hydrogen permeation fluctuation on the micro-damage of the structure.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas storage and transportation, and in particular to a method for studying the elastic-plastic mechanical behavior of a weld microstructure under a 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 affect the normal production of the gas field and 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 maximum 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 refers to the micron to nanometer damage in the structure, including crystal defects, material performance degradation and micron-level 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 and then causes 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 propagation is basically proposed for a certain type of metal material or a certain deformation stage, which cannot be universally applied. This paper proposes a method for studying the elastic-plastic mechanical behavior of the microstructure of the weld under large-span temperature difference, aiming to obtain the distribution law of the mechanical performance parameters of the weld under micro damage under the coupling of large-span alternating temperature difference and hydrogen permeation, and provide a dynamic mechanism for the subsequent crack propagation and residual strength evaluation of high-pressure vessel welds. Summary of the invention

[0005] The purpose of the present invention is to provide a method for studying the elastic-plastic mechanical behavior of weld microstructure 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: A method for studying the elastic-plastic mechanical behavior of weld microstructure under large-span temperature difference comprises the following steps: Step 1: Establish a large-span temperature difference working condition simulation test bench; Step 2: Based on the large-span temperature difference working condition simulation test bench described in step 1, and combined with nanoindentation and mechanical tensile tests at different angles, multi-scale mechanical experiments are carried out to study the evolution of the stress-strain curve of the weld microstructure under large-span alternating temperature difference, and establish a local elastic-plastic model of the weld microstructure under large-span alternating temperature difference based on multi-factor coupling; Step 3: Establish and solve the local elastic-plastic model of the weld microstructure under the large-span alternating temperature difference obtained in step 2 and the coordinated dynamic model of the induced stress field near the crack tip to analyze the influence of operating pressure, temperature difference and hydrogen permeation fluctuation on its structural micro-damage; Step 4: Combine the influence of fluctuations such as operating pressure, temperature difference and hydrogen permeation on the micro-damage of the structure obtained in step 3 to determine the micro-damage-time history relationship of the high-pressure vessel weld; Step 5: Compress the micro-damage-time history relationship obtained in step 4, analyze the elastoplastic mechanical behavior of the tip microstructure of the micro-damage-induced crack in the weld under large-span alternating temperature difference, and give the main controlling factors and changing laws.

[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 nanoindentation in step 2 is measured by a nanoindentation tester, and the mechanical tensile test in step 2 is performed by a tensile testing machine.

[0011] Compared with the prior art, the beneficial effect of the present invention is that the elastic-plastic mechanical behavior research method of the weld microstructure under large-span temperature difference can obtain the orthotropic elastic parameters and crystal plastic parameters of a single grain in the weld matrix under large-span temperature difference, and combine with the material mechanics theory to establish a crystal plasticity model, predict the anisotropic yield criterion of the weld under large-span temperature difference, and compare it with the actual tensile curve at different angles to complete the model correction, thereby obtaining the distribution law of the mechanical performance parameters of the weld under micro-damage under the coupling effect of large-span alternating temperature difference and hydrogen permeation, providing a dynamic mechanism for the subsequent high-pressure container weld crack propagation and residual strength evaluation. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 A method block diagram of a method for studying the elastic-plastic mechanical behavior of a weld microstructure under a 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 a method for studying the elastic-plastic mechanical behavior of a weld microstructure under a large-span temperature difference disclosed in an embodiment of the present invention; Figure 3 The present invention is a block diagram of an experimental method of an experimental bench for studying the elastic-plastic mechanical behavior of a weld microstructure under a large-span temperature difference disclosed in an embodiment of the present 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 3The present invention provides a technical solution: a method for studying the elastic-plastic mechanical behavior of a weld microstructure under a large-span temperature difference, comprising the following steps: step one: establishing a large-span temperature difference working condition simulation test bench; step two: based on the large-span temperature difference working condition simulation test bench of step one, and combining nanoindentation and mechanical tensile experiments at different angles, multi-scale mechanical experiments are performed to study the evolution of the stress-strain curve of the weld microstructure under a large-span alternating temperature difference, and to establish a local elastic-plastic model of the weld microstructure under a large-span alternating temperature difference based on multi-factor coupling; step three: using the large-span alternating temperature difference obtained in step two to obtain the local elastic-plastic model of the weld microstructure under a large-span alternating temperature difference; and step four: using the large-span alternating temperature difference obtained in step two to obtain the local elastic-plastic model of the weld microstructure under a large-span alternating temperature difference. The local elastic-plastic model of the weld microstructure under the alternating temperature difference and the coordinated dynamic model of the induced stress field near the crack tip are established and solved, and the influence of operating pressure, temperature difference and hydrogen permeation fluctuations on its structural microdamage is analyzed; Step 4: Combined with the influence of operating pressure, temperature difference and hydrogen permeation fluctuations on its structural microdamage obtained in step 3, the microdamage-time history relationship of the high-pressure vessel weld is determined; Step 5: Compress the microdamage-time history relationship obtained in step 4, analyze the elastic-plastic mechanical behavior of the tip microstructure of the crack induced by microdamage of the weld under large-span alternating temperature difference, and give the main controlling factors and changing laws.

[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 3 The present invention provides a technical solution: a method for studying the elastic-plastic mechanical behavior of a weld microstructure under a 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 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 to a nitrogen discharge port, the large-span temperature difference working condition simulation test bench includes the 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, and the liquid nitrogen tank stores liquid nitrogen The liquid nitrogen tank is connected to the vaporizer, one of the reheaters is connected to the vaporizer, and is connected to the nitrogen protection shell through the regulating valve 1, and is connected to the constant temperature and pressure gas buffer tank through the regulating valve 2. The liquid nitrogen tank is connected to the constant temperature and pressure gas buffer tank through the regulating valve 3, and the constant temperature and pressure gas buffer tank is connected to the low temperature zone through the regulating valve 5. The acid gas tank stores acid gas, another reheater is connected to the acid gas tank, and is connected to the constant temperature and pressure gas buffer tank through the regulating valve 4, and the constant temperature and pressure gas buffer tank is connected to the high temperature zone through the regulating valve 6.

[0017] 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.

[0018] 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.

[0019] See also Figure 1 - Figure 3 The present invention provides a technical solution: a method for studying the elastic-plastic mechanical behavior of a weld microstructure under a large-span temperature difference, wherein the nanoindentation in step two is measured by a nanoindentation tester, and the mechanical tensile test in step two is performed by a tensile testing machine.

[0020] An embodiment of the present invention further utilizes a large-span temperature difference working condition simulation test bench to manufacture weld micro-damage, and uses a nanoindentation tester to obtain the unloading curve and loading curve of the weldment, and uses the unloading curve to inversely calculate the elastic parameters, and uses the loading curve to inversely calculate the plastic parameters, and predicts the tensile curve based on the two parameters and the crystal plasticity model. At the same time, a tensile testing machine is used to actually stretch the weldment at angles of 0 degrees, 45 degrees, and 90 degrees, and the crystal plasticity model is compared with the actual tensile curve to complete the model correction.

Claims

1. A method for studying the elastic-plastic mechanical behavior of weld microstructure under large-span temperature difference, characterized in that: The following steps are involved: Step 1: Establish a large-span temperature difference working condition simulation test bench; Step 2: Based on the large-span temperature difference working condition simulation test bench described in step 1, and combined with nanoindentation and mechanical tensile tests at different angles, multi-scale mechanical experiments are carried out to study the evolution of the stress-strain curve of the weld microstructure under large-span alternating temperature difference, and establish a local elastic-plastic model of the weld microstructure under large-span alternating temperature difference based on multi-factor coupling; Step 3: Establish and solve the local elastic-plastic model of the weld microstructure under the large-span alternating temperature difference obtained in step 2 and the coordinated dynamic model of the induced stress field near the crack tip to analyze the influence of operating pressure, temperature difference and hydrogen permeation fluctuation on its structural micro-damage; Step 4: Combine the influence of fluctuations such as operating pressure, temperature difference and hydrogen permeation on the micro-damage of the structure obtained in step 3 to determine the micro-damage-time history relationship of the high-pressure vessel weld; Step 5: Compress the micro-damage-time history relationship obtained in step 4, analyze the elastoplastic mechanical behavior of the tip microstructure of the micro-damage-induced crack in the weld under large-span alternating temperature difference, and give the main controlling factors and changing laws.

2. The method for studying the elastic-plastic mechanical behavior of weld microstructure 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 method for studying the elastic-plastic mechanical behavior of weld microstructure 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 method for studying the elastic-plastic mechanical behavior of weld microstructure under large-span temperature difference according to claim 1 is characterized in that: The nanoindentation in step 2 is measured by a nanoindentation tester, and the mechanical tensile test in step 2 is performed by a tensile testing machine.

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