A method for predicting stress corrosion susceptibility of tube-tube sheet welded joints in shell-and-tube heat exchangers

Abstract

The present invention belongs to the technical field of welding materials and is a method for predicting the stress corrosion sensitivity of the welded joints between tubes and tube sheets of shell and tube heat exchangers. The influence of cracks on the redistribution of welding residual stress is taken into account, and the degree of agreement between the finite element simulation results of the stress distribution of the heat exchanger tube head and the actual component stress distribution is improved, thereby accurately obtaining the influence of crack defects on the stress concentration of the tube head. Through corrosion tests on different areas of the welded joints between tubes and tube sheets, the electrochemical corrosion parameters of different areas are obtained. Advantages: Based on the thermodynamic and electrochemical theories of materials, the stress corrosion sensitivity of the welded joints between tubes and tube sheets is predicted, and the stress corrosion sensitivity of different parts is obtained, which has important theoretical guiding significance and engineering application value for ensuring the safe operation of the heat exchanger tube heads.

Description

Technical Field

[0001] The invention belongs to the technical field of welding materials and relates to the prediction of stress corrosion sensitivity of welding parts. Background Art

[0002] Heat exchangers are unit thermal equipment in process industries. They are key equipment for balancing process heat or recovering process waste heat or cooling materials. They are widely used in oil refining, chemical, energy, metallurgy, pharmaceutical, food and other industries. In the petrochemical industry, heat exchangers account for 1 / 3 of the total equipment and about 1 / 4 of the equipment investment. Shell and tube heat exchangers account for more than 80% of the total number of heat exchangers in industrial devices due to their rotating shell structure, which is safe, reliable and durable.

[0003] With the development of science and technology, shell and tube heat exchangers are gradually developing towards large-scale and high-parameterization. The welding connection between the heat exchange tube and the tube sheet is the weak link of the shell and tube heat exchanger and one of the bottlenecks restricting the long-term safe operation of the device. During the welding process, the drastic and uneven heat distribution leads to differences in the microstructure and composition of different areas of the tube head, resulting in large residual plastic deformation and residual stress. The heat exchanger tube head is particularly susceptible to the influence of the surrounding environment during service, and it is inevitable that there will be structural defects. The most common defect is the crack defect. The presence of cracks will destroy the continuity of the base material, reduce the bearing capacity of the structure, and produce stress and strain concentration around the defect. When the corrosive medium exists, the welded joint between the tube and the tube sheet is prone to stress corrosion cracking. At present, the influence of welding residual stress on the welded joint with crack defects does not take into account the distribution of actual welding residual stress, and the research on its influence on the tube head of the heat exchanger with crack defects is relatively lacking. The prediction of material stress corrosion sensitivity is mainly focused on uniform materials, and the prediction of stress corrosion sensitivity in different welding areas is rarely reported. Due to the particularity of the welded joint structure between shell and tube tubes and tube sheets, the stress distribution is relatively complex, which increases the difficulty of predicting the stress corrosion sensitivity of the tube head.

[0004] The presence of cracks in the heat exchanger tube head will destroy its structural integrity, making it more susceptible to stress corrosion cracking under the influence of the environment and load. Due to the effect of welding thermal cycles, the microstructure and mechanical properties of different regions of the material change, and the impact of cracks in different regions on the reliability of the tube head varies. Welding residual stress is an important factor affecting crack initiation and cracking, and it has an important influence on crack initiation sensitivity. At present, the research on the sensitivity of stress corrosion cracking of heat exchanger tube heads has the following shortcomings:

[0005] (1) The influence of cracks on residual stress of welded joints does not take into account the distribution of real residual stress. Most of them apply residual stress by presetting the temperature field, which has a certain deviation from the actual working conditions.

[0006] (2) The corrosion sensitivity of heat exchanger materials is mainly concentrated on the corrosion behavior of the tube and tube sheet base materials, and there is a lack of research on the corrosion sensitivity of different welding areas.

[0007] (3) The research on stress corrosion behavior of heat exchanger tube heads mainly stays on failure analysis, and there are no relevant reports on the effects of crack defects, welding residual stress and microstructure on the stress corrosion sensitivity of heat exchanger tube heads.

[0008] Therefore, from the perspective of structure, material and environment, it is particularly important to propose a stress corrosion sensitivity prediction method for the welded joints between tubes and tube sheets of shell and tube heat exchangers. Accurate stress corrosion sensitivity prediction of heat exchanger tube heads with crack defects can improve the reliability of heat exchanger tube heads, which has important theoretical guidance significance and engineering application value. Summary of the invention

[0009] The object of the present invention is to provide a method for predicting the stress corrosion sensitivity of a welded joint between a tube and a tube sheet of a shell and tube heat exchanger.

[0010] The technical solution of the present invention is: a method for predicting stress corrosion sensitivity of a tube-tube sheet welded joint of a shell-and-tube heat exchanger, (1) determining the geometric dimensions of the tube-tube sheet welded joint of the shell-and-tube heat exchanger;

[0011] (2) Determine the material parameters of the heat exchanger tube head that change with temperature, including: elastic modulus, Poisson's ratio, yield strength, thermal conductivity, thermal expansion coefficient, specific heat, and density;

[0012] (3) Establish a two-dimensional axisymmetric finite element model of the welded joint between the tube and tube sheet of a shell-and-tube heat exchanger, set the boundary constraints of the tube head of the shell-and-tube heat exchanger, set the X-axisymmetric constraint at the tube end point I of the heat exchange tube, and set the Y-axisymmetric constraint at the plate end point II of the tube sheet; calculate the welding residual stress distribution of the heat exchanger tube head and obtain the welding residual stress distribution diagram;

[0013] (4) The mesh of the finite element model is kept unchanged, and cracks are introduced using the extended finite element method to import welding residual stress. The welding residual stress before and after the introduction does not change significantly; considering the influence of cracks on the residual stress evolution of the heat exchanger tube head, the welding residual stress redistribution diagram of the heat exchanger tube head is obtained;

[0014] (5) Heat treatment is performed on the welded joints between the heat exchanger tubes and the tube sheets to eliminate stress, and stress-free corrosion specimens are processed. The corrosion current density and corrosion potential of the welded joints of the specimens are measured to obtain the electrochemical corrosion parameters of different welding areas of the tube heads;

[0015] (6) The connection between load and corrosion reaction is established through the activation energy of chemical reaction. During stress corrosion, the anodic current density of material activation and dissolution is closely related to the Gibbs free energy:

[0016] (1)

[0017] In formula (1), i 0 is the anode current density of the material when there is no external load, △ G σ is the change in Gibbs energy caused by the external load, R is the gas constant, T is the absolute temperature;

[0018] In the micro-region at the crack tip, stress has little effect on the electron activity on the material surface. Therefore, the effect of the applied load on the anodic activated dissolution ignores the effect of the electron work function, and the Gibbs free energy is expressed as:

[0019] (2)

[0020] In formula (2), △ U is the change in internal energy of the material under load, △ S is the entropy change, P is the pressure, V is the change in volume under load;

[0021] During the plastic deformation of the material, the volume change caused by dislocation is small, and the volume change caused by plastic deformation is ignored. V It is expressed as:

[0022] (3)

[0023] In formula (3), M is the atomic mass of the metal, r is the material density, E is the elastic modulus of the material, v is Poisson's ratio, s h is the hydrostatic pressure;

[0024] s h It is expressed by the principal stress at the crack tip:

[0025] (4)

[0026] In formula (4), s 1. s 2. s 3 represents the principal stresses in the three directions of the crack tip (the corresponding relationship between the three directions is related to the established model. The principal stress corresponding directions of the three-dimensional coordinate system model generally refer to the X, Y and Z directions. For the heat exchanger tube head, it usually refers to the radial, axial and circumferential directions);

[0027] In the material electrode, the amount of entropy change and internal energy change is represented by the plastic strain energy density stored in the electrode:

[0028] (5)

[0029] In formula (5), △ U is the change in internal energy of the material under load, △ S is the entropy change, W is the atomic mass of the material, r is the density of the metal electrode, W P is the plastic strain energy density per unit molar volume of the material, or is the ratio of the density of plastic strain energy stored in the material;

[0030] According to the Ramberg-Osgood power law: (6)

[0031] In formula (6), P and n are material constants, s is the total stress;

[0032] Under the action of external load, the plastic strain energy density per unit molar volume of the material is expressed as:

[0033] (7)

[0034] In formula (7), s i for i The principal stress in the direction e pi for i The plastic strain in the direction P and n are material constants, s 1. s 2. s 3 represents the principal stress at the crack tip;

[0035] The complex stress at the crack tip of the welded joint between the tube and the tube sheet of a shell and tube heat exchanger promotes the anodic dissolution of the tube head. The stress corrosion sensitivity of different welding areas of the heat exchanger tube head is predicted based on the anodic activation dissolution current density under load.

[0036] (7) Change the position of the crack in the heat exchanger tube head and repeat steps (3) to (6) to obtain the stress corrosion sensitivity of different welding areas of the shell and tube heat exchanger tube and tube sheet welded joint.

[0037] Preferably, a two-dimensional axisymmetric finite element model is established with the aid of ABAQUS finite element software.

[0038] The beneficial effects of the present invention are as follows: (1) The present invention takes into account the actual residual stress distribution of the heat exchanger tube head, adopts the extended finite element method to introduce cracks into the welded joints between the heat exchanger tube and the tube sheet, accurately establishes a heat exchanger tube head model containing crack defects, and ensures the accuracy of the finite element model in calculating local mechanical parameters.

[0039] (2) The present invention can obtain the electrochemical corrosion characteristics of different welding areas of the heat exchanger tube head by means of electrochemical testing, thereby ensuring the correctness of the electrochemical corrosion parameters of the material.

[0040] (3) The present invention is based on thermodynamics and electrochemical theories and proposes the method of the present invention, which has important theoretical guiding significance and application value for ensuring the long-term safe operation of the heat exchanger tube head. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic diagram of a two-dimensional model of a welded joint between a heat exchanger tube and a tube sheet in the method of the present invention;

[0042] Figure 2 Schematic diagram of residual stress distribution of the welded joint between the heat exchanger tube and the tube sheet in the method of the present invention;

[0043] Figure 3 Schematic diagram of residual stress distribution of a heat exchanger tube head containing crack defects in the method of the present invention;

[0044] Figure 4 Schematic diagram of the redistribution of residual stress in the heat exchanger tube head welding in the method of the present invention;

[0045] Figure 5 It is a flowchart of the method of the present invention.

[0046] In the figure: 1-heat exchange tube, 2-tube sheet, 3-welded joint, 4-tube end point I, 5-plate end point II. DETAILED DESCRIPTION

[0047] (1) Determine the geometric dimensions of the welded joints between the tubes and tube sheets of the shell and tube heat exchanger.

[0048] (2) Determine the material parameters of the heat exchanger tube head that change with temperature, including: elastic modulus, Poisson's ratio, yield strength, thermal conductivity, thermal expansion coefficient, specific heat, and density.

[0049] (3) In order to improve the calculation efficiency, the ABAQUS finite element software is used to establish a two-dimensional axisymmetric finite element model of the tube and tube sheet welding joint of the shell and tube heat exchanger, as shown in Figure 1 As shown. Set the boundary constraints of the tube head of the shell and tube heat exchanger. Set the X-axis symmetry constraint at the tube end point I of the heat exchange tube, and set the Y-axis symmetry constraint at the plate end point II of the tube sheet. Calculate the welding residual stress distribution of the heat exchanger tube head, as shown in Figure 2 shown.

[0050] (4) When using the traditional method, after the crack is introduced into the finite element model, the mesh of the model changes, and it is difficult to import the welding residual stress. After the welding residual stress is imported, there is a certain difference from the model without defects. Therefore, the extended finite element method is used to introduce cracks without changing the mesh of the finite element model. There is no obvious change in the welding residual stress before and after the introduction. Figure 3 Considering the influence of cracks on the residual stress evolution of the heat exchanger tube head, the redistribution of welding residual stress of the heat exchanger tube head is obtained, which is more in line with engineering practice, such as Figure 4 shown.

[0051] (5) The welding thermal cycle causes different microstructures in different welding areas of the heat exchanger tube head, resulting in different corrosion behaviors in different welding areas. The welded joints between the heat exchanger tube and the tube sheet were heat treated to eliminate stress, and stress-free corrosion specimens were processed. The corrosion current density and corrosion potential of the welded joints of the specimens were measured to obtain the electrochemical corrosion parameters of different welding areas of the tube head.

[0052] (6) In working conditions, when load and corrosive medium coexist, the stress corrosion cracking sensitivity of the material is affected by the dual effects of corrosion and stress. According to thermodynamics and electrochemical theory, the connection between load and corrosion reaction is established through the activation energy of chemical reaction. In the process of stress corrosion, the anodic current density of material activation and dissolution is closely related to the Gibbs free energy:

[0053] (1)

[0054] In formula (1), i 0 is the anode current density of the material when there is no external load, △ G σ is the change in Gibbs energy caused by the external load, R is the gas constant, T is the absolute temperature. In the micro-region at the crack tip, the stress has little effect on the electron activity on the material surface. Therefore, the effect of the applied load on the anodic activated dissolution is ignored, and the Gibbs free energy can be expressed as:

[0055] (2)

[0056] In formula (2), △ U is the change in internal energy of the material under load, △ S is the entropy change, P is the pressure, V is the volume change under load. During the plastic deformation of the material, the volume change caused by dislocation is small. Therefore, ignoring the volume change caused by plastic deformation, △ VIt can be expressed as:

[0057] (3)

[0058] In formula (3), M is the atomic mass of the metal, r is the material density, E is the elastic modulus of the material, v is Poisson's ratio, s h is the hydrostatic pressure. s h It can be expressed by the principal stresses in three directions at the crack tip:

[0059] (4)

[0060] In formula (4), s 1. s 2. s 3 represents the principal stress at the crack tip;

[0061] In the material electrode, the amount of entropy change and internal energy change can be expressed by the plastic strain energy density stored in the electrode:

[0062] (5)

[0063] In formula (5), △ U is the change in internal energy of the material under load, △ S is the entropy change, W is the atomic mass of the material, r is the density of the metal electrode, W P is the plastic strain energy density per unit molar volume of the material, or is the ratio of the plastic strain energy density stored in the material.

[0064] According to the Ramberg-Osgood power law: (6)

[0065] In formula (6), P and n are material constants, s is the total stress.

[0066] Under the action of external load, the plastic strain energy density per unit molar volume of the material can be expressed as:

[0067] (7)

[0068] In formula (7), s i for i The principal stress in the direction e pi for i The plastic strain in the direction P and n are material constants, s 1. s 2. s 3 represents the principal stress at the crack tip;

[0069] The complex stress at the crack tip of the welded joint between the tube and the tube sheet of a shell and tube heat exchanger promotes the anodic dissolution of the tube head. The stress corrosion sensitivity of different welding areas of the heat exchanger tube head can be predicted based on the anodic activation dissolution current density under load.

[0070] (7) By changing the position of the crack in the heat exchanger tube head and repeating steps (3) to (6), the stress corrosion sensitivity of different welding areas of the shell and tube heat exchanger tube and tube sheet welded joint can be obtained.

[0071] The process of the present invention is as follows Figure 5 shown.

[0072] The key technical points of the invention are as follows:

[0073] (1) For the local structural discontinuity components of the tube and tube sheet welded joints of shell and tube heat exchangers, the present invention proposes a method for predicting the stress corrosion sensitivity of the tube and tube sheet welded joints, which can predict the stress corrosion sensitivity of different welding areas of the heat exchanger tube head.

[0074] (2) The extended finite element method is used to accurately introduce the welding residual stress and cracks of the complete structure into the tube head model. Considering the influence of cracks on the evolution of residual stress in the heat exchanger tube head welding, the residual stress redistribution of the tube head model with crack defects is obtained, ensuring the accuracy of the calculation of local mechanical parameters at the crack tip.

[0075] (3) The corrosion behaviors of different welding areas of the heat exchanger tube head were measured respectively, and the electrochemical corrosion parameters of different welding areas of the heat exchanger tube head were obtained, which provided more accurate data for the prediction of stress corrosion sensitivity of the heat exchanger tube head.

[0076] (4) Based on thermodynamic and electrochemical theories, the effects of crack defects, welding residual stress and microstructure on the stress corrosion sensitivity of the heat exchanger tube head are fully considered, which improves the accuracy of the prediction of the stress corrosion sensitivity of the heat exchanger tube head.

Claims

1. A method for predicting stress corrosion sensitivity of tube-tube sheet welded joints in a shell-and-tube heat exchanger, characterized by: (1) Determine the geometric dimensions of the welded joints between the tube and tube sheet of a shell and tube heat exchanger; (2) Determine the material parameters of the heat exchanger tube head that change with temperature, including: elastic modulus, Poisson's ratio, yield strength, thermal conductivity, thermal expansion coefficient, specific heat, and density; (3) Establish a two-dimensional axisymmetric finite element model of the welded joint between the tube and tube sheet of a shell-and-tube heat exchanger, set the boundary constraints of the tube head of the shell-and-tube heat exchanger, set the X-axisymmetric constraint at the tube end point I of the heat exchange tube, and set the Y-axisymmetric constraint at the plate end point II of the tube sheet; calculate the welding residual stress distribution of the heat exchanger tube head and obtain the welding residual stress distribution diagram; (4) The mesh of the finite element model is kept unchanged, and cracks are introduced using the extended finite element method to import welding residual stress. The welding residual stress before and after the introduction does not change significantly; considering the influence of cracks on the residual stress evolution of the heat exchanger tube head, the welding residual stress redistribution diagram of the heat exchanger tube head is obtained; (5) Heat treatment is performed on the welded joints between the heat exchanger tubes and the tube sheets to eliminate stress, and stress-free corrosion specimens are processed. The corrosion current density and corrosion potential of the welded joints of the specimens are measured to obtain the electrochemical corrosion parameters of different welding areas of the tube heads; (6) The connection between load and corrosion reaction is established through the activation energy of chemical reaction. During stress corrosion, the anodic current density of material activation and dissolution is closely related to the Gibbs free energy: (1) In formula (1), i 0 is the anode current density of the material when there is no external load, △ G σ is the change in Gibbs energy caused by the external load, R is the gas constant, T is the absolute temperature; In the micro-region at the crack tip, the stress has little effect on the electron activity on the material surface. Therefore, the effect of the applied load on the anodic activation dissolution ignores the effect of the electron work function. The change in Gibbs energy caused by the applied load is expressed as: (2) In formula (2), △ U is the change in internal energy of the material under load, T is the absolute temperature, S is the entropy change, Q is the pressure, V is the change in volume under load; During the plastic deformation of the material, the volume change caused by dislocation is small, and the volume change caused by plastic deformation is ignored. V It is expressed as: (3) In formula (3), M is the atomic mass of the metal, v is Poisson's ratio, σ h is the hydrostatic pressure, E is the elastic modulus of the material, ρ is the density of the metal electrode; σ h It is expressed by the principal stress at the crack tip: (4) In formula (4), σ 1. σ 2. σ 3 represents the principal stresses in three directions of the crack tip; In the material electrode, the amount of entropy change and internal energy change is represented by the plastic strain energy density stored in the electrode: (5) In formula (5), △ U is the change of material internal energy under load, T is the absolute temperature, △ S is the entropy change, W is the atomic mass of the material, ρ is the density of the metal electrode, η is the ratio of the plastic strain energy density stored in the material, W P is the plastic strain energy density per unit molar volume of the material; According to the Ramberg-Osgood power law: (6) In formula (6), ε p is the plastic strain, P and n are material constants, σ is the total stress; Under the action of external load, the plastic strain energy density per unit molar volume of the material is expressed as: (7) In formula (7), σ i for i The principal stress in the direction ε pi for i The plastic strain in the direction P and n are material constants, σ 1. σ 2. σ 3 represents the principal stresses in three directions of the crack tip; The complex stress at the crack tip of the welded joint between the tube and the tube sheet of a shell and tube heat exchanger promotes the anodic dissolution of the tube head. The stress corrosion sensitivity of different welding areas of the heat exchanger tube head is predicted based on the anodic activation dissolution current density under load. (7) Change the position of the crack in the heat exchanger tube head and repeat steps (3) to (6) to obtain the stress corrosion sensitivity of different welding areas of the shell and tube heat exchanger tube and tube sheet welded joint.

2. A method for predicting stress corrosion sensitivity of a tube-tube sheet welded joint of a shell and tube heat exchanger according to claim 1, characterized in that: A two-dimensional axisymmetric finite element model was established with the assistance of ABAQUS finite element software.

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