Method for regulating and controlling residual stress of weld joint of crude oil storage tank

Abstract

The invention relates to a crude oil storage tank weld joint residual stress regulation and control method, which comprises the following steps: carrying out residual stress test through an indentation method, a profile method, a neutron diffraction test and the like, and verifying and obtaining a welding residual stress distribution rule of several different welding structures of a storage tank; and finite element calculation regulation and control are conducted on the welding residual stress of the key welding structure position, the optimal local heat treatment scheme is obtained by researching numerical simulation analysis of the welding residual stress under different heat treatment working conditions, and the purpose of effectively eliminating the welding residual stress is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of petroleum, and particularly relates to a method for regulating the residual stress of the weld seam of a crude oil storage tank. Background Art

[0002] The large-scale development of petroleum storage tanks will become an inevitable trend. Due to the large structural size of the storage tanks, a large number of splicing welds are required. During the welding process, welding residual stress will be generated, which is likely to form stress concentration, accelerate stress fatigue and corrosion damage, cause changes in the structural size and shape, and even crack and fail.

[0003] In the existing technical solutions, for petroleum storage tanks, the focus is mostly on optimizing the materials used in crude oil storage tanks. For example, the patent with the publication number CN111607737A discloses a high-strength steel plate for large crude oil storage tanks and its production method, which limits the composition of the steel plate, adds strengthening elements inside the steel plate, and performs certain post-treatment, so that the steel plate has good strength.

[0004] The patent with the publication number CN115725899A discloses a corrosion-resistant steel and its preparation method and application, as well as a crude oil storage tank. This patent also limits the composition of the corrosion-resistant steel and improves the performance of the heat-affected zone of the weld.

[0005] However, in the above-mentioned solutions, they are all general solutions. For each crude oil storage tank, due to different structures, sizes, weld positions, and welding methods, the stress concentration positions and stress magnitudes generated by the welding of each crude oil storage tank are also different. Simply providing steel plates with good general effects cannot solve the problems of different stress concentration positions or magnitudes caused by different structural sizes of each crude oil storage tank. Summary of the Invention

[0006] The purpose of the present invention is to overcome the above-mentioned shortcomings of the existing technology and provide a method and device for regulating the residual stress of the weld seam of a crude oil storage tank, so as to solve the problem that it is difficult to specifically solve the residual thermal stress of the weld seam of each crude oil storage tank in the existing technology.

[0007] To achieve the above purpose, the present invention adopts the following technical solutions:

[0008] A method for regulating the residual stress of the weld seam of a crude oil storage tank includes the following steps:

[0009] S110, processing a storage tank specimen according to the size and welding process of the crude oil storage tank;

[0010] S120. Establish a finite element model for the storage tank specimen to simulate the welding process of the storage tank specimen; determine the boundary conditions by simulating the mechanical property parameters and thermal physical property parameters of the storage tank specimen during the welding process through finite element simulation; calculate the changes in the real-time welding stress and stress field of the entire storage tank specimen during the welding process through thermo-elastoplastic analysis, and obtain the distribution law of the welding residual stress along the thickness and height.

[0011] S130. Conduct residual stress testing on the storage tank specimen to obtain the residual stress test results of the storage tank specimen.

[0012] S140. Verify the distribution law of the welding residual stress along the thickness and height through the residual stress test results of the storage tank specimen; if the verification is successful, determine the weld with the maximum residual stress as the key position, simulate the heat treatment conditions of the key position through the finite element model, and obtain the optimal heat treatment plan when the residual stress is the lowest under a certain heat treatment condition.

[0013] A further improvement of the present invention lies in:

[0014] Preferably, in S110, the storage tank specimen is processed from several materials according to the structure of the crude oil storage tank.

[0015] Preferably, in S120, when establishing the finite element model and performing geometric mesh division on the storage tank specimen, the following steps are included:

[0016] (1) Establish a local two-dimensional axisymmetric model according to the size and structure of the storage tank specimen.

[0017] (2) Perform mesh division on the local two-dimensional axisymmetric model, set the mesh size of the weld and the heat affected zone as the first density size, set the mesh size far from the weld as the second density size, and use transitional meshes for division in the middle part, where the first density size is smaller than the second density size.

[0018] (3) Adopt two-dimensional four-node tetrahedron reduced integration elements, and use the element passivation and activation method to form the weld metal. Preferably, in S120, the process of simulating the mechanical property parameters and thermal physical property parameters of the storage tank specimen during the welding process includes:

[0019] (1) During the temperature field calculation process, set the absolute zero temperature and the ambient temperature; assign the ambient temperature to the model during welding calculation, assign the preheating temperature to the model during heat treatment calculation, and simultaneously set the convective heat dissipation and radiative heat dissipation of the model.

[0020] (2) Fix both ends of the storage tank specimen according to the actual welding situation to prevent movement, and calculate the boundary conditions of the stress field.

[0021] Preferably, in S120, the process of obtaining the distribution law of the welding residual stress along the thickness and height is:

[0022] Carry out simulation of butt welding of the edge plate, simulation of girth welding with variable wall thickness for the first and second circumferences of the tank wall, simulation of longitudinal welding of the first circumference of the tank wall, carry out simulation of longitudinal welding of the first circumference of the tank wall, carry out simulation of variable wall thickness welding of the edge plate and the middle plate of the tank bottom plate, carry out simulation of the butt weld of the middle plate of the tank bottom plate, carry out simulation of the T-shaped weld between the tank wall and the tank bottom plate, and obtain the distribution law of welding residual stress along the thickness and height.

[0023] Preferably, the residual stress test results include the residual stress on the structure surface, the residual stress in the thickness direction, and the residual stress of each cross-section.

[0024] Preferably, the objects for residual stress testing of the crude oil storage tank include the V-shaped butt test plate of the edge plate, the K-shaped butt test plate of the circumferential weld of the wall plate, the X-shaped groove butt test plate of the longitudinal weld of the wall plate, the V-shaped butt test plate of the middle plate, the V-shaped butt test plate of the edge plate - middle plate, and the T-shaped fillet weld test plate of the large corner joint.

[0025] Preferably, the V-shaped butt test plate of the edge plate uses indentation method, neutron diffraction and contour method to test the welding residual stress; the remaining test plates are selected to use the indentation method for testing.

[0026] Preferably, in S140, the heat treatment conditions of the simulated key positions include the holding temperature, the cooling rate, and the width of the heating zone.

[0027] Preferably, the key position is the T-shaped weld.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The present invention relates to a method for regulating the welding residual stress of a crude oil storage tank, including carrying out residual stress testing through methods such as indentation method, contour method, and neutron diffraction testing, verifying and obtaining the distribution law of welding residual stress of several different welding structures of the storage tank, and carrying out finite element calculation and regulation on the welding residual stress at the key welding structure positions. Through numerical simulation analysis of the welding residual stress under different heat treatment conditions, the optimal local heat treatment plan is obtained to achieve the purpose of effectively eliminating the welding residual stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic flow chart of a method for regulating the welding residual stress of a crude oil storage tank according to an embodiment of the present invention;

[0031] Figure 2 It is a schematic diagram of the mesh division of the present invention;

[0032] Figure 3 It is a diagram of the parameter setting of the edge plate butt weld model of the present invention;

[0033] Figure 4Schematic diagram of grid division for the butt weld of the edge plate of the present invention;

[0034] Figure 5 Schematic diagram of grid division for the first and second circumferential welds of the present invention;

[0035] Figure 6 Contour map of residual stress for the first and second circumferential welds of the present invention;

[0036] Among them, figure (a) is the contour map of residual stress for the first circumferential weld; figure (b) is the contour map of residual stress for the second circumferential weld.

[0037] Figure 7 Schematic diagram of grid division for the first and second longitudinal welds of the present invention;

[0038] Among them, figure (a) is the first circle; figure (b) is the second circle.

[0039] Figure 8 Contour map of residual stress for the first and second longitudinal welds of the present invention;

[0040] Among them, figure (a) is the first circle; figure (b) is the second circle.

[0041] Figure 9 Schematic diagram of the overall and local longitudinal weld models of the present invention;

[0042] Among them, figure (a) is the overall view; figure (b) is the enlarged local view;

[0043] Figure 10 Schematic diagram of grid division for the overall and local longitudinal welds of the present invention;

[0044] Among them, figure (a) is the overall view; figure (b) is the enlarged local view;

[0045] Figure 11 Schematic diagram of the welding model of the bottom plate and the edge plate of the present invention;

[0046] Among them, figure (a) is the overall view; figure (b) is the enlarged local view;

[0047] Figure 12 Schematic diagram of grid division for the welding of the bottom plate and the edge plate of the present invention;

[0048] Among them, figure (a) is the overall view; figure (b) is the enlarged local view;

[0049] Figure 13 Contour map of residual stress for the weld between the bottom plate and the edge plate of the present invention;

[0050] Figure (a) is the normal stress in the S11 direction; figure (b) is the normal stress in the S33 direction.

[0051] Figure 14Schematic diagram of the weld between the web plates in the present invention;

[0052] Among them, figure (a) is the overall view; figure (b) is the partially enlarged view;

[0053] Figure 15 Schematic diagram of the mesh division of the weld between the web plates in the present invention;

[0054] Among them, figure (a) is the overall view; figure (b) is the partially enlarged view;

[0055] Figure 16 Schematic diagram of the T-shaped weld between the tank wall and the tank bottom in the present invention;

[0056] Among them, figure (a) is the overall view; figure (b) is the partially enlarged view;

[0057] Figure 17 Schematic diagram of the mesh division of the T-shaped weld between the tank wall and the tank bottom in the present invention;

[0058] Among them, figure (a) is the overall view; figure (b) is the partially enlarged view;

[0059] Figure 18 Contour map of the residual stress of the T-shaped weld between the tank wall and the tank bottom in the present invention;

[0060] In figure (a), it is the normal stress in the S11 direction, in figure (b) it is the normal stress in the S22 direction, and in figure (c) it is the normal stress in the S33 direction;

[0061] Figure 19 Comparison result of the finite element simulation and the experimental test in the present invention;

[0062] Among them, in figure (a), it is the axial welding residual stress of the inner surface path 1; in figure (b), it is the circumferential welding residual stress of the inner surface path 1; in figure (c), it is the axial welding residual stress of the outer surface path 2; in figure (d), it is the circumferential welding residual stress of the outer surface path 2.

[0063] Figure 20 Schematic diagram of the regulation result of the heat treatment at the insulation temperature of 600 °C in the present invention;

[0064] Figure 21 Schematic diagram of the partial contour map of the regulation result of the heat treatment at the insulation temperature of 600 °C in the present invention;

[0065] Figure 22 Schematic diagram of the regulation result of the heat treatment at the insulation temperature of 650 °C in the present invention. Detailed implementation manners

[0066] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0067] As Figure 1 As shown in the schematic flow chart of a method for heat treatment regulation of residual stress of T-shaped welds in crude oil storage tanks according to an embodiment of the present invention, a method for heat treatment regulation of residual stress of T-shaped welds in crude oil storage tanks includes the following steps:

[0068] S110. Process storage tank specimens according to the size and welding process of the crude oil storage tank. The storage tank specimens are processed from several different materials and different structures respectively.

[0069] S120. Perform finite element modeling on the sizes and structures of the storage tank specimens, simulate the welding process of the storage tank specimens, obtain the finite element calculation results of the residual stress of the welded structure of the storage tank specimens, and use the residual stress test results to verify the finite element calculation results of the residual stress to verify the accuracy of the finite element results.

[0070] Specifically, S120 includes:

[0071] Perform mesh division on the geometric model of the storage tank specimen, and after obtaining the mechanical property parameters and thermal physical property parameters of the material temperature change of each storage tank specimen during the welding process, set boundary conditions, and use thermo-elastoplastic analysis to calculate the change process of the real-time welding stress and strain fields during the welding process. Use the elastoplastic mechanics model and calculate the welding stress field using the incremental theory.

[0072] Specifically, the performing mesh division on the geometric models of the storage tank specimens specifically includes:

[0073] (1) Establish a two-dimensional axisymmetric model according to the size and structure of each storage tank specimen;

[0074] (2) Use finite element software to perform mesh division on the local two-dimensional axisymmetric model, set the mesh size of the weld and the heat-affected zone to the first density size, set the mesh size far from the weld to the second density size, and use transition meshes for division in the middle part, where the first density size is smaller than the second density size;

[0075] (3) Adopt two-dimensional four-node tetrahedron reduced integration elements, and use the element passivation and activation method to form the weld metal.

[0076] Specifically, after obtaining the mechanical property parameters and thermophysical property parameters of the material temperature change of each of the storage tank specimens, the boundary conditions for temperature field calculation are carried out, specifically including:

[0077] (1) During the temperature field calculation process, set the absolute zero temperature and the ambient temperature. Assign the ambient temperature to the model during welding calculation, and assign the preheating temperature to the model during heat treatment calculation. At the same time, set the convective heat dissipation and radiative heat dissipation of the model to ensure the normal change of the model temperature;

[0078] (2) Stress field calculation boundary conditions: Fix the two ends of each of the storage tank specimens according to the actual welding situation to prevent movement.

[0079] Specifically, perform simulation through a finite element calculation model, specifically including:

[0080] Perform simulation of the butt welding of the edge plates, perform simulation of the variable wall thickness circumferential welding of the first and second circumferences of the tank wall, perform simulation of the longitudinal welding of the first circumference of the tank wall, perform simulation of the variable wall thickness welding of the edge plate and the middle plate of the tank bottom plate, perform simulation of the butt weld of the middle plate of the tank bottom plate, and perform simulation of the T-shaped weld of the tank wall and the tank bottom plate to obtain the distribution law of the welding residual stress along the thickness and height.

[0081] S130. Test all the storage tank specimens to obtain the residual stress test results of each storage tank specimen. The residual stress test results include the residual stress on the inner surface of the structure at the weld position, the residual stress on the outer surface, the residual stress in the thickness direction of each cross-section, and the residual stress of each cross-section.

[0082] Perform residual stress testing on the storage tank specimens to obtain the residual stress distribution law of the crude oil storage tank, specifically including:

[0083] Perform welding residual stress testing on the welding test plates on the storage tank specimens. The welding test plates include edge plate V-groove butt test plates, wall plate circumferential weld K-groove butt test plates, wall plate longitudinal weld X-groove butt test plates, middle plate V-groove butt test plates, edge plate - middle plate V-groove butt test plates, and large fillet weld T-joint test plates. Among them, the edge plate V-groove butt test plates use the indentation method, neutron diffraction, and contour method to test the welding residual stress, and the remaining test plates are tested by the indentation method.

[0084] Verify the simulation results of S129 through the residual stress test results of S130. If the trends are the same, if it is qualified, execute S140. If the established simulation model is unqualified, repeat step S120 to re-establish a finite element simulation model.

[0085] S140.

[0086] Determine the key positions of the crude oil storage tank according to the residual stress distribution law of the storage tank specimen, the finite element calculation results of the residual stress, and the residual stress test results. The key positions are the positions with the maximum stress. Perform finite element calculation on the key positions, and the position with the maximum stress among the determined key positions is the local key position. Simulate the heat treatment conditions of the local key position to obtain the optimal heat treatment plan.

[0087] Perform finite element calculation on the key positions according to the following, simulate the local heat treatment conditions, and obtain the optimal heat treatment plan, which specifically includes:

[0088] Simulate the local heat treatment conditions by respectively performing heat treatment on the key positions for the holding temperature, cooling rate, and heating band width, and determine the optimal heat treatment plan for the key positions.

[0089] It should be understood that the embodiments in the present application test the welding residual stress of the storage tank welding specimen. According to the actual size and welding process of the storage tank, 5-7 kinds of welding specimens with different materials and structures are processed, and then the indentation method, contour method, and neutron diffraction test are used to measure the residual stress on the surface, thickness direction, and entire cross-section of the structure, providing test verification for finite element calculation.

[0090] Perform finite element modeling and analysis according to the actual size and structure of the storage tank to obtain the finite element calculation results of the residual stress of 5-7 kinds of welding structures, and conduct comparative analysis with the test results.

[0091] Field residual stress test of crude oil storage tank. Select a crude oil storage tank area, and conduct the residual stress indentation method test on site to test the residual stress distribution law at different positions and different welding structures (typical structures such as fillet welds, bottom plate butt / lap joints, etc.).

[0092] Finite element regulation of welding residual stress of crude oil storage tank. Perform finite element calculation on the key positions of the storage tank according to the actual size and structure, and obtain the optimal heat treatment plan by simulating different local heat treatment conditions.

[0093] Among them, the material parameters adopted for the geometric model and mesh generation through finite element calculation are: 12MnNiVR / Q235B; the materials of the edge plates of the 1st to 6th circumferences of the tank wall and the tank bottom are 12MnNiVR, and the material of the middle plate of the tank bottom is Q235B. In order to save calculation time, a local two-dimensional axisymmetric model is established according to the actual size and structure of the storage tank. The finite element software ABAQUS is used to generate the mesh for the established geometric model. The mesh size of the weld and the heat affected zone is small and the distribution is dense, while the mesh size far from the weld is large and the distribution is sparse. The middle part is divided using transitional meshes to ensure the calculation accuracy. The aspect ratio of the mesh of the entire model is controlled within 1:10, and the aspect ratio of the mesh at the weld is controlled within 1:5. The mesh type adopts the two-dimensional four-node tetrahedron reduced integration element (CAX4R) with high calculation accuracy. The element deactivation and activation method is used to form the weld metal. In the weld and the heat affected zone, the residual stress is relatively large and the mesh division is dense, while in the area far from the weld, the mesh is sparse. Figure 2 The mesh generation pattern is given, and the same elements and nodes are used for thermal analysis and force analysis. The mesh generation methods for the geometric models under various different weld forms and heat treatment conditions are the same.

[0094] The mechanical property parameters of 12MnNiVR / Q235B at different temperatures are obtained through experimental tests and referring to relevant literature, mainly including elastic modulus, tensile strength and yield strength. The thermodynamic properties of the material are all related to temperature, and it is assumed that the physical properties of the material remain unchanged at high temperatures (above the melting point). Tables 1-1 to 1-4 respectively give the mechanical property parameters and thermal physical property parameters of 12MnNiVR / Q235B varying with temperature.

[0095] Table 1-1 Thermal Physical Property Parameters of 12MnNiVR

[0096]

[0097] Table 1-2 Mechanical Property Parameters of 12MnNiVR

[0098]

[0099] Table 1-3 Thermal Physical Property Parameters of Q235B

[0100]

[0101] Table 1-4 Mechanical Properties of Q235B

[0102]

[0103] Boundary conditions for temperature field calculation: During the temperature field calculation, set the absolute zero temperature of -273.15 °C and the ambient temperature. Assign the ambient temperature to the model during welding calculation and the preheating temperature to the model during heat treatment calculation. At the same time, set the convective heat dissipation and radiative heat dissipation of the model, as Figure 3 , to ensure the normal change of the model temperature.

[0104] Boundary conditions for stress field calculation: Fix both ends of the model according to the actual welding situation to prevent rigid body movement and approach the actual situation to the greatest extent.

[0105] The difference in the thermomechanical properties between the welding consumables and the base material results in non-uniform elastoplastic deformation inside the structure during the welding process, and residual stresses will be generated inside the structure after welding. Currently, the theories regarding welding residual stresses and deformations mainly include: thermoplastic analysis, viscoplastic analysis, considering the coupling effect of phase transformation and thermal stress, and the inherent strain method. For thermo-elastoplastic analysis, generally, the sequential coupling method is mainly used to solve the thermal stress and strain through the thermal strain behavior during the welding thermal cycle. First, calculate the welding temperature field and use the temperature field as the predefined field for calculating the residual stress, and then calculate the residual stress field. Using thermo-elastoplastic analysis calculation can grasp the change process of the stress / strain field of the welded part in real time.

[0106] When calculating the residual stress, non-linear factors such as materials and geometry make the calculation of welding residual stress more complex and difficult. Therefore, to ensure the accuracy of the solution, regard the welding thermal stress field as a transient problem of material non-linearity, calculate the elastoplastic mechanics model using the incremental theory, and make the following assumptions:

[0107] 1) The yield of the welded part obeys the Von Mises yield criterion;

[0108] 2) The behavior in the plastic zone of the welded part obeys the plastic flow criterion and the isotropic hardening criterion;

[0109] 3) The elastoplastic strain and the thermal strain are inseparable;

[0110] 4) The thermal physical properties and stress / strain related to temperature change linearly within a small time increment.

[0111] The Von Mises yield criterion believes that the main reason for the material flow failure is the shape change energy, that is, it is considered that whether it is a multi-axial stress state or a uniaxial stress state, the shape change energy is the main factor leading to the material flow failure. In the uniaxial stress state, if the stress reaches the yield limit σ S , the welded part will show flow failure, and at this time, the shape change energy can be calculated as follows:

[0112]

[0113] According to the Mises yield criterion, under a multiaxial stress state, when the shape change energy of the material reaches the shape change energy of the above-mentioned uniaxial stress state, the welded part will undergo flow failure. Therefore, the flow failure condition under complex states is as follows:

[0114]

[0115] where σ1, σ2, and σ3 are the principal stresses in three orthogonal directions, and σ S is the yield limit under uniaxial tension.

[0116] At this time, define as the equivalent stress. When the value of the equivalent stress reaches or even exceeds the yield limit of the welded part, the welded part yields and undergoes plastic deformation.

[0117] Assume that after the welded part enters the yield state, it is gradually loaded in small increments, and the strain components are divided into two parts: elastic strain increment and plastic strain increment:

[0118] {dε} = {dε} e + {dε} p (1 - 3)

[0119] The derivative of the vector {σ}, and ζ is a scalar factor.

[0120] The flow criterion can be geometrically interpreted as the direction of the plastic strain increment vector of the welded part being consistent with the normal direction of the yield surface. Therefore, it is sometimes also called the normal flow criterion.

[0121] Problems such as material nonlinearity and geometric nonlinearity are difficult points in the calculation process of the welding residual stress field. In finite element analysis, the welding thermal stress field is regarded as a transient problem of material nonlinearity for research. In the calculation, an elastoplastic mechanical model is selected, and the incremental theory is used to calculate the welding stress field.

[0122] (1) Stress-strain relationship

[0123] Elastic region:

[0124] The total strain increment can be expressed as:

[0125] {dε} = {dε} e + {dε} T (1 - 6)

[0126] {dε} e is the elastic strain increment, {dε} T is the thermal strain increment. Since the elastic matrix [D] e changes with temperature, the elastic strain increment {dε} e can be expressed as

[0127]

[0128] {dε} T is the incremental differential of {α0T} (where α0 is the coefficient of linear expansion at the initial temperature), i.e.,

[0129]

[0130] Substituting (2 - 6) and (2 - 7) into (2 - 5), the incremental stress - strain relationship in the elastic region is obtained as

[0131]

[0132] Since the coefficient of linear expansion is related to temperature, its effective value can be expressed as

[0133]

[0134] Plastic region:

[0135] In the plastic region, assume the yield condition of the material is

[0136] f(σ) = f0(ε p , T) (1 - 11)

[0137] where f is the yield function; f0 is the yield stress function related to temperature and plastic strain. The total strain increment in the plastic region can be expressed as:

[0138] {dε} = {dε} p + {dε} e + {dε} T (1 - 12)

[0139] where the plastic strain increment {dε} p can be further expressed according to the plastic flow rule as

[0140]

[0141] Therefore, the stress - strain relationship in the plastic region is:

[0142]

[0143] where D ep is the elastoplastic matrix

[0144]

[0145] (2) Equilibrium equation

[0146] From the stress - strain relationship {dσ} = [D]{dε}-{C}dT and according to the virtual displacement principle, the equilibrium equation can be obtained

[0147]

[0148] where [B] is the geometric matrix related to the geometry of the element.

[0149] Since it is in a state of equilibrium at time t, so

[0150] {dF} e = ∫∫ Δv [B] T {σ}dV (1-17)

[0151] Then (2-15) can be written as

[0152] {dF} e = ∫∫ Δv [B] T ([D]{dε}-{C}dT)dV (1-18)

[0153] Or

[0154] {dF} e +{dR} e = [K] e {dδ} (1-19)

[0155] The equivalent nodal force of initial strain {dR} e = ∫∫ Δv [B] T {C}dTdV

[0156] The element stiffness matrix [K] e = ∫∫ Δv [B] T [D][B]dV

[0157] The algebraic equations for solving the nodal displacements are

[0158] [K]{dδ} = {dF} (1-20)

[0159] where [K] = ∑[K] e , {dF} = ∑({dF e +dR e})

[0160] Considering that there is generally no external force during the welding process, taking ∑{dF} e as 0, so

[0161] {dF} = ∑{dR} e (1-21)

[0162] (3) Solve the equations

[0163] When analyzing thermo-elastoplastic problems using the finite element method, first, the component to be analyzed is divided into a finite number of analysis elements, and then the temperature increments calculated from the temperature field are gradually applied. The essence of the finite element method analysis and solution is to gradually transform the non-linear stress-strain relationship during the loading process into a linear problem for analysis and processing. The load during the welding process is mainly caused by the temperature change ΔT. Thus, the temperature change from T to T + ΔT calculated from the temperature field analysis is divided into several incremental loads and gradually applied to the structure for solution.

[0164] Finite element calculations were performed on six typical welding structures of a 12MnNiVR storage tank to obtain the distribution laws of welding residual stresses along the height, thickness, and cross-section. These were compared with the residual stress data obtained from indentation method tests to verify the effectiveness of the finite element calculations.

[0165] The results calculated using ABAQUS are default generated in the global Cartesian coordinate system. The stress components S11, S22, and S33 represent the stresses in the X, Y, and Z directions respectively, and U1, U2, and U3 represent the displacement deformations in the X, Y, and Z directions respectively.

[0166] Simulation of butt welding of the edge plate

[0167] The material of the storage tank edge plate is 12MnNiVR. Modeling was carried out according to the actual size and structure of the storage tank. The model is as Figure 4 shown. There are a total of 2 welds, and transitional meshing is used for division as Figure 5 shown. There are a total of 3600 meshes, and the entire welding process is simulated using birth and death elements.

[0168] The maximum value of the transverse residual tensile stress after butt welding of the edge plate is 536 MPa, located at the weld center. The maximum value of the residual compressive stress is 694 MPa, located at the connection between the lower surface of the test plate and the backing plate. The maximum value of the longitudinal residual tensile stress is 770 MPa, located at the weld. The maximum value of the longitudinal residual compressive stress is 230 MPa, located at the connection between the lower surface of the test plate and the backing plate.

[0169] Simulation of the circumferential welding of the first / second ring of the tank wall with variable wall thickness

[0170] The materials of the first and second rings of the tank wall are both 12MnNiVR. Modeling was carried out according to the actual size and structure of the storage tank. There are a total of 5 welds. The model is as Figure 6 shown. Transitional meshing is used for division as Figure 7 shown. There are a total of 7793 meshes, and the entire welding process is simulated using birth and death elements.

[0171] The residual stresses after the welding simulation of the first and second circumferential welds are as Figure 8As shown, the radial residual stress is relatively small and can be ignored. The maximum axial residual tensile stress is approximately 525 MPa, located on the outer surface of the weld. The maximum axial residual compressive stress is approximately 619 MPa, located on the middle side of the weld. The maximum circumferential residual tensile stress is 748 MPa, located at the weld, and the maximum circumferential residual compressive stress is 142 MPa, located at both ends of the inner side of the weld.

[0172] Simulation of the first circumferential longitudinal weld of the tank wall

[0173] The base material of the first circumferential longitudinal weld of the tank wall is 12MnNiVR. Modeling is carried out according to the actual size and structure of the storage tank. The model is as Figure 9 shown. There are a total of 5 welds, and transitional meshing is used for division as Figure 10 shown. There are a total of 2587 meshes, and the birth and death elements are used to simulate the entire welding process.

[0174] The maximum value of the axial residual tensile stress is 743 MPa, located at the edge of the weld; the maximum axial residual compressive stress is 112 MPa, located on both sides of the weld at the center of the tank wall. The maximum value of the circumferential residual tensile stress is 500 MPa, located in the heat-affected zone near the weld edge; the maximum value of the circumferential residual compressive stress is 540 MPa, located at the center of the tank wall.

[0175] Simulation of the variable-wall-thickness welding of the edge plate and the middle plate of the tank bottom

[0176] The material of the tank bottom edge plate is 12MnNiVR, and the material of the middle plate is Q235B. Modeling is carried out according to the actual size and structure of the storage tank. The model is as Figure 11 shown. There are a total of 2 welds, and transitional meshing is used for division as Figure 12 shown. There are a total of 2226 meshes, and the birth and death elements are used to simulate the entire welding process.

[0177] The residual stress after the simulation of the welding of the bottom edge and the edge plate is as Figure 13 shown. The maximum value of the transverse residual tensile stress is 550 MPa, located on the edge plate near the weld edge. The maximum value of the transverse residual compressive stress is 298 MPa, located on the inner side of the weld near the backing plate; the maximum value of the longitudinal residual tensile stress is 718 MPa, located at the weld and near the edge plate, and the maximum value of the longitudinal residual compressive stress is 53.1 MPa, located at the bottom of the edge plate and near the weld.

[0178] Simulation of the butt weld of the middle plate of the tank bottom

[0179] The material of the middle plate of the tank bottom is Q235B. Modeling is carried out according to the actual size and structure of the storage tank. The model is as Figure 14 shown. There are a total of 2 welds, and transitional meshing is used for division as Figure 15 shown. There are a total of 2189 meshes, and the birth and death elements are used to simulate the entire welding process.

[0180] After the intermediate plate is welded to the intermediate plate, the maximum value of the transverse residual tensile stress is 273 MPa, which is located at the upper surface of the test plate restricted by the backing plate. The maximum value of the residual compressive stress is 178 MPa, which is located at the lower surface of the test plate restricted by the backing plate. The maximum value of the longitudinal residual tensile stress is 253 MPa, which is located at the upper surface of the test plate near the weld edge and most of the weld interior is residual tensile stress. The maximum value of the longitudinal residual compressive stress is 54.5 MPa, which is located at the lower surface of the test plate restricted by the backing plate.

[0181] Simulation of the T-shaped weld between the tank wall and the tank bottom plate

[0182] The materials of the first ring of the tank wall and the edge plate of the tank bottom plate are 12MnNiVR. Modeling is carried out according to the actual size and structure of the storage tank. The model is as Figure 16 shown. There are a total of 7 welds, and transitional meshes are used for division as Figure 17 shown. There are a total of 9142 meshes, and the birth and death elements are used to simulate the entire welding process.

[0183] The residual stress after welding the T-shaped weld between the tank wall and the bottom plate is as Figure 18 shown. The maximum value of the radial residual tensile stress is 563 MPa, which is located at the contact between the last weld and the bottom plate. The maximum residual compressive stress is located near the weld in the bottom plate. The maximum value of the axial residual tensile stress is 542 MPa, which is located at the contact between the weld and the tank wall. The maximum value of the axial residual compressive stress is 346 MPa, which is located near the weld in the tank wall. The maximum value of the circumferential residual tensile stress is 794 MPa, which is located at the last weld and all of the weld interior is residual tensile stress. The maximum value of the circumferential residual stress compressive stress is 171 MPa, which is located near the weld in the bottom plate.

[0184] Finite element calculations are carried out on different welding structures of the storage tank to obtain the stress nephograms of various welding structures, and then the distribution laws of the welding residual stress along the thickness and height are obtained. Among various different welding structures, the residual stress of the T-shaped weld is the largest and is also the key object of concern in engineering. Therefore, finite element research on the heat treatment of the T-shaped weld is carried out to obtain the optimal heat treatment plan for regulating its welding residual stress.

[0185] Welding residual stress tests are carried out on 6 kinds of welded test plates processed, namely the edge plate V-shaped butt joint test plate, the wall plate circumferential weld K-shaped butt joint test plate, the wall plate longitudinal weld X-shaped groove butt joint test plate, the intermediate plate V-shaped butt joint test plate, the edge plate-intermediate plate V-shaped butt joint test plate, and the large fillet weld T-shaped corner joint test plate. Among them, the edge plate V-shaped butt joint test plate uses the indentation method, neutron diffraction, and contour method to test the welding residual stress, and the remaining test plates are tested by the indentation method.

[0186] Principle of measuring residual stress by the indentation method

[0187] In the indentation method test, a certain load is applied to the point to be measured on the specimen through an indenter, causing plastic deformation on the surface of the specimen to form a spherical crown indentation, and a certain stress field is generated around the indentation. During the formation of the stress field, a certain strain increment will be caused around the indentation. The formation of the stress field and the generation of the strain increment are not only related to the diameter D of the steel ball, the load P, the distance L from this point to the center of the load action, and the material properties of the specimen, but also closely related to the welding residual stress. Therefore, when the diameter of the steel ball, the load, and the distance are certain, the calibration relationship between the welding residual stress and the strain increment is established. During the actual residual stress test, the residual stress in the specimen is determined by obtaining the strain increment during the indentation formation process.

[0188] The KJS-3 type indentation method stress test system is adopted. The system includes two parts: stress test and indentation manufacturing. The stress test part is the KJS-3 type indentation stress tester, which uses the windows CE operating system, can automatically zero, measure, and display data in real time. The indentation manufacturing part includes an indentation striking device, a fixed base, and a centering microscope. The fixed base can directly adsorb on the surface of magnetic materials to fix the centering microscope or the indentation striking device. The centering microscope with a magnification of 10 times can quickly find the striking position of the strain gauge. The indentation striking device uses a spring as a power storage device to push a 1 / 16-inch hard alloy spherical indenter to strike on the workpiece to manufacture an indentation.

[0189] The KJS-3 type indentation stress tester can not only measure strain but also measure stress, and the maximum measurement error is less than 15 - 20 MPa. It has the advantages of small volume, light weight, convenient and accurate testing. It is mainly used for experimental stress analysis and non-destructive measurement of residual stress in on-site welded structures, and is widely used in industries such as material research, mechanical manufacturing, mining and metallurgy, water conservancy projects, railway transportation, chemical equipment, shipbuilding, aviation industry, and power construction. It is an important tool for non-destructive static strain-stress measurement and analysis in research and design, production and construction by industrial and mining enterprises, scientific research institutes, and universities.

[0190] Principle of Residual Stress Measurement by Neutron Diffraction

[0191] The basic principle of neutron diffraction measurement of residual stress is the same as the method of X-ray diffraction measurement of stress. When a neutron beam with a wavelength of λ passes through a polycrystalline material sample, the change in the crystal lattice spacing under the action of stress is △d, and the change in the diffraction peak angle is △θ. According to the Bragg relationship:

[0192] nλ=2dsinθ(3-1)

[0193] In the formula: n is an integer, and 2θ is the diffraction angle

[0194] The basic principle of neutron diffraction testing is as follows: when there is stress in the material, the lattice plane spacing will change. By measuring the shift of the diffraction peak position 2θ of the sample, the change in the lattice plane spacing d in a certain direction of the material relative to the lattice plane spacing d0 in the stress-free state is calculated to obtain the strain ε.

[0195]

[0196] In the formula: θ0 is the Bragg diffraction peak position of the sample in the stress-free state.

[0197] When the principal strains in three directions are measured, according to Hooke's law, the three principal stresses σ can be calculated by conversion. i =

[0198]

[0199] In the formula: i = x, y, z, corresponding to three orthogonal directions respectively; Ehkl is the elastic modulus; vhkl is the Poisson's ratio.

[0200] Principle of measuring residual stress by the contour method

[0201] The component is completely cut into two halves along the plane where the stress needs to be studied and evaluated. Due to stress release, the contour of the cutting surface deforms. Assuming that the deformed contour of the cutting surface is caused by the elastic release of residual stress, if an external force is applied to restore the deformed cutting surface to the plane state before cutting, the obtained stress state is equivalent to the original residual stress on this plane before cutting. Therefore, the original internal stress value can be obtained by using the deformed contour on the cutting surface.

[0202] The implementation of the contour method is divided into the following three steps.

[0203] (1) Cutting. Under the restraint state, the sample is cut into two halves along the stress evaluation position by wire cutting. Due to stress release, the cutting surface will produce elastic deformation.

[0204] (2) Surface contour measurement and fitting. A high-precision contour measurement device is used to measure the deformed contour of the cutting surface. The contour data of the two cutting surfaces are averaged and a smooth surface fitting is performed to eliminate measurement errors.

[0205] (3) Stress reconstruction analysis. Taking the measured contour as the boundary condition, the stress state after the deformed cutting surface is restored to the plane state before cutting (or the cutting surface in the plane state is changed to the deformed contour in the opposite direction) is analyzed by elastic finite element method. Assuming that no plastic deformation occurs, the stress on the obtained cutting surface is equivalent to the original stress at the same position before cutting.

[0206] Compare and analyze the test photos of the V-groove butt joint test plate for the edge plate, the K-groove butt joint test plate for the circumferential seam of the wall plate, the X-groove butt joint test plate for the longitudinal seam of the wall plate, the V-groove butt joint test plate for the middle plate, the V-groove butt joint test plate for the edge plate - middle plate, and the T-joint test plate for the large fillet weld.

[0207] The distribution of the welding residual stress on the upper surface of the edge plate butt joint test plate. Due to the cooling shrinkage of the outermost weld, tensile stress is generated. The transverse welding residual stress has the maximum tensile stress at the weld toe, and the stress value in the middle of the weld is relatively small. The stress value first forms the maximum tensile stress at the weld toe as the distance from the weld center increases, and then decreases.

[0208] The comparison results between the contour method test and the finite element are as Figure 19 shown. Longitudinal residual stress data is extracted from the upper surface path of the test plate respectively. The stress distribution and trend of the results of the contour method are basically the same as those of the finite element calculation.

[0209] For the welding residual stress test of the circumferential welds with variable wall thickness in the first and second laps of the tank wall, the circumferential welding residual stress on both the inner and outer surfaces shows an M-shaped distribution, that is, there is the maximum axial tensile stress at the weld toe; the circumferential welding residual stress shows a near-character shape distribution, and the circumferential welding residual stress is located at the weld. Through the indentation method test and verification, the finite element analysis can better reflect its welding residual stress distribution.

[0210] The test results of the welding residual stress on the outer surface of the longitudinal weld in the first lap of the tank wall are as Figure 19 shown. The axial welding residual stress shows a near-character shape distribution, and the maximum axial tensile stress is at the weld; the circumferential welding residual stress shows an M-shaped distribution, and there is the maximum circumferential tensile stress at the weld toe, and the stress value gradually decreases as the distance increases.

[0211] The comparison of the welding residual stress test for the variable wall thickness welding of the edge plate and the middle plate. Due to the discontinuity caused by the variable wall thickness welding and the different material parameters on both sides, there is a large gap in the stress values on both sides of the weld. The maximum value of the transverse welding residual stress is located at the weld toe near the edge plate, and there is also a tensile stress peak on the middle plate side but not the maximum value; the longitudinal welding residual stress as a whole still shows a near-character shape distribution, but the longitudinal welding residual stress value on the middle plate side is higher than that on the edge plate side, and the maximum value is still located at the weld.

[0212] For the welding residual stress of the butt joint of the middle plate of the tank bottom plate, the transverse welding residual stress on the upper surface shows an M-shaped distribution, and the maximum transverse tensile stress is located at the weld toes on both sides of the weld. As the distance from the weld center increases, the transverse welding residual tensile stress first decreases and then increases; the longitudinal welding residual stress shows a near-character shape distribution, and the maximum value is located at the weld. As the distance from the weld center increases, the longitudinal welding residual stress gradually decreases.

[0213] Comparison of T - type welding residual stresses in the tank wall and bottom plate. Due to the severe structural discontinuity and the irregularity of the weld beads, three axial tensile stress peaks are formed on both the inner and outer surfaces of the T - type weld, namely at the connection between the weld and the bottom plate, at the connection between the welds, and at the connection between the weld and the tank wall. The maximum axial tensile stress is generated at the connection between the tank wall and the weld on both the inner and outer surfaces; the circumferential welding residual stress has a maximum value at the weld and generally presents a shape similar to the Chinese character "ji".

[0214] Based on the above simulation and test results, the simulation of heat treatment regulation for the residual stress of the T - type weld is carried out.

[0215] According to the results of finite - element calculation and the results of residual stress testing, it can be obtained that the residual stress at the T - type weld is the largest, and this position is also the most dangerous in engineering practice. Therefore, this position is selected for finite - element heat treatment regulation to reduce the welding residual stress of the T - type joint.

[0216] Influence of holding temperature

[0217] According to GB / T 30583 - 2014 "Code for Post - weld Heat Treatment of Pressure - bearing Equipment", its minimum heat treatment temperature is determined to be 600 °C. At the same time, its Ac1 = 738 °C. According to the heat treatment requirements, the holding temperature needs to be < Ac1. Therefore, the holding temperatures are selected as 600, 650, and 700 °C.

[0218] (1) Holding temperature 600 °C

[0219] The results after heat treatment with a holding temperature of 600 °C are as Figure 20 shown. The radial residual stress decreases from 563 MPa to 438 MPa, the axial residual stress decreases from 542 MPa to 361 MPa, and the circumferential residual stress decreases from 794 MPa to 640 MPa. The residual stresses are all improved and reduced.

[0220] The nephogram of the local key position after heat treatment is as Figure 21 shown. The radial stress shows concentrated tensile stress at the connection between the weld and the bottom plate, and compressive stress is formed inside the bottom plate near the weld; the axial stress forms concentrated tensile stress at the connection between the weld and the tank wall, and compressive stress is formed inside the tank wall near the weld; the circumferential stress is mainly concentrated at the weld, and the tensile stress of the outer weld is greater than that of the inner weld.

[0221] Holding temperature 650 °C

[0222] Carry out heat treatment with a holding temperature of 650 °C. The results after heat treatment are as Figure 22As shown, the radial residual stress decreases from 563 MPa to 328 MPa, the axial residual stress decreases from 542 MPa to 246 MPa, the circumferential residual stress decreases from 794 MPa to 700 MPa. The decrease in the radial and axial residual stresses is greater than that at 600 °C, but the decrease in the circumferential residual stress is reduced. The overall residual stress is improved and reduced.

[0223] The local nephogram after heat treatment shows that concentrated tensile stress appears at the connection between the weld and the bottom plate in the radial stress, and compressive stress is formed inside the bottom plate near the weld; concentrated tensile stress is formed at the connection between the weld and the tank wall in the axial stress, and compressive stress is formed inside the tank wall near the weld at the same time; the circumferential stress is mainly concentrated at the weld, and the tensile stress of the outer weld is greater than that of the inner weld.

[0224] Insulation temperature: 700 °C

[0225] When heat treatment is carried out at an insulation temperature of 700 °C, the radial residual stress decreases from 563 MPa to 241 MPa, the axial residual stress decreases from 542 MPa to 172 MPa, and the circumferential residual stress decreases from 794 MPa to 674 MPa. The decrease in the radial, axial, and circumferential residual stresses is greater than that at 600 °C and 650 °C, and the overall residual stress in all directions is improved and reduced.

[0226] After heat treatment, concentrated tensile stress appears at the connection between the weld and the bottom plate in the radial stress, and compressive stress is formed inside the bottom plate near the weld. The radial stress value is significantly improved compared with that at 600 °C and 650 °C; concentrated tensile stress is formed at the connection between the weld and the tank wall in the axial stress, and compressive stress is formed inside the tank wall near the weld at the same time. The axial stress value is significantly improved compared with that at 600 °C and 650 °C; the circumferential stress is mainly concentrated at the weld, and the tensile stress of the outer weld is greater than that of the inner weld. The circumferential stress value is significantly improved compared with that at 600 °C and 650 °C.

[0227] Since the circumferential residual stress is the largest, the circumferential residual stresses of Path 1 and Path 2 after being regulated at different insulation temperatures are extracted for comparison. The residual stress after heat treatment at an insulation temperature of 700 °C is the lowest. Therefore, an insulation temperature of 700 °C is selected for subsequent heat treatment to explore the optimal heat treatment plan.

[0228] Influence of the cooling rate

[0229] In order to avoid a large temperature difference caused by too high a heating rate and at the same time meet the requirement that the minimum heating / cooling rate > 55 °C / h, heat loading rates of 56 °C / h, 76 °C / h, 96 °C / h, 106 °C / h, and 136 °C / h are selected.

[0230] Cooling rate: 56 °C / h

[0231] With an insulation temperature of 700°C and a cooling rate of 56°C / h, after heat treatment, the radial residual stress decreased from 563 MPa to 151 MPa, the axial residual stress decreased from 542 MPa to 110 MPa, and the circumferential residual stress decreased from 794 MPa to 417 MPa. The overall residual stress in all directions improved and decreased.

[0232] After heat treatment, the local nephogram shows that concentrated tensile stress appears at the connection between the weld and the bottom plate in the radial stress, and compressive stress forms inside the bottom plate near the weld. Concentrated tensile stress forms at the connection between the weld and the tank wall in the axial stress, and compressive stress forms inside the tank wall near the weld at the same time. The circumferential stress decreases significantly, mainly located at the discontinuity of the weld toe.

[0233] Cooling rate: 76°C / h

[0234] With an insulation temperature of 700°C and a cooling rate of 76°C / h, after heat treatment, the radial residual stress decreased from 563 MPa to 150 MPa, the axial residual stress decreased from 542 MPa to 107 MPa, and the circumferential residual stress decreased from 794 MPa to 459 MPa. The overall residual stress in all directions improved and decreased.

[0235] After heat treatment, the local nephogram shows that concentrated tensile stress appears at the connection between the weld and the bottom plate in the radial stress, and compressive stress forms inside the bottom plate near the weld. Concentrated tensile stress forms at the connection between the weld and the tank wall in the axial stress, and compressive stress forms inside the tank wall near the weld at the same time. The circumferential stress decreases significantly, mainly located at the discontinuity of the weld toe.

[0236] Cooling rate: 96°C / h

[0237] With an insulation temperature of 700°C and a cooling rate of 96°C / h, the results after heat treatment are as Figure 4-13 shown. The radial residual stress decreased from 563 MPa to 166 MPa, the axial residual stress decreased from 542 MPa to 118 MPa, and the circumferential residual stress decreased from 794 MPa to 479 MPa. The overall residual stress in all directions improved and decreased.

[0238] Concentrated tensile stress appears at the connection between the weld and the bottom plate in the radial stress, and compressive stress forms inside the bottom plate near the weld. Concentrated tensile stress forms at the connection between the weld and the tank wall in the axial stress, and compressive stress forms inside the tank wall near the weld at the same time. The circumferential stress decreases significantly, mainly located at the discontinuity of the weld toe.

[0239] Cooling rate: 106°C / h

[0240] With a holding temperature of 700 °C and a cooling rate of 106 °C / h, after heat treatment, the radial residual stress decreased from 563 MPa to 166 MPa, the axial residual stress decreased from 542 MPa to 118 MPa, and the circumferential residual stress decreased from 794 MPa to 479 MPa. The overall residual stress in all directions was improved and reduced.

[0241] After heat treatment, the local nephogram shows that the radial stress has concentrated tensile stress at the connection between the weld and the bottom plate, and compressive stress is formed inside the bottom plate near the weld. The axial stress has concentrated tensile stress at the connection between the weld and the tank wall, and compressive stress is formed inside and outside the tank wall near the weld. The circumferential stress has decreased significantly, mainly located at the discontinuity of the weld toe.

[0242] With a holding temperature of 700 °C and a cooling rate of 136 °C / h, after heat treatment, the radial residual stress decreased from 563 MPa to 143 MPa, the axial residual stress decreased from 542 MPa to 99 MPa, and the circumferential residual stress decreased from 794 MPa to 521 MPa. The overall residual stress in all directions was improved and reduced.

[0243] After heat treatment, the local nephogram shows that the radial stress has concentrated tensile stress at the connection between the weld and the bottom plate, and compressive stress is formed inside the bottom plate near the weld. The axial stress has concentrated tensile stress at the connection between the weld and the tank wall and inside the tank wall, and compressive stress is formed inside and outside the tank wall near the weld. The circumferential stress has decreased significantly, mainly located on the inner wall.

[0244] The circumferential residual stresses of Path 1 and Path 2 after being regulated with different cooling rates were compared. The results show that the residual stress is the lowest after heat treatment with a cooling rate of 56 °C / h. Therefore, a holding temperature of 56 °C / h was selected for subsequent heat treatment exploration.

[0245] Influence of heating band width

[0246] According to the uniform temperature zone HB = 7nhk (1 < n < 3), different heating band widths were set, which were 200 mm, 400 mm, 600 mm, 800 mm, 1000 mm, and 1200 mm along the inner and outer walls of the tank respectively. Based on the welding simulation of the T-shaped weld of the 2.5-section tank wall and tank bottom plate, heat treatment simulation was carried out to study the most suitable heating band width for the T-shaped weld.

[0247] (1) Heat treatment results with a 200-mm heating band

[0248] A heating band with a width of 200 mm was set, and heat treatment was carried out by forced convection.

[0249] The residual tensile stress at the heat-treated weld is greatly reduced and transferred to the tank wall and bottom at a certain distance from the weld. The radial and axial residual stresses of the T-shaped weld are reduced to about 100 MPa, and the circumferential residual stress is reduced to about 400 MPa. At the same time, a partial enlarged view of the weld is output. There is still welding residual tensile stress only at the geometric discontinuity, but the stress value has also been greatly reduced.

[0250] (2) Heat treatment results with a 300 mm heating band

[0251] The residual tensile stress at the heat-treated weld is greatly reduced and transferred to the tank wall and bottom at a certain distance from the weld. The radial and axial residual stresses of the T-shaped weld are reduced to about 150 MPa and 110 MPa respectively, and the circumferential residual stress is reduced to about 425 MPa. At the same time, a partial enlarged view of the weld is output. There is still welding residual tensile stress only at the geometric discontinuity.

[0252] The residual tensile stress at the heat-treated weld is greatly reduced and transferred to the tank wall and bottom at a certain distance from the weld. The radial and axial residual stresses of the T-shaped weld are reduced to about 150 MPa and 110 MPa respectively, and the circumferential residual stress is reduced to about 418 MPa. At the same time, a partial enlarged view of the weld is output. There is still welding residual tensile stress only at the geometric discontinuity, but the stress value has also been greatly reduced.

[0253] (4) Heat treatment results with a 600 mm heating band

[0254] The heat treatment results show that the residual tensile stress at the weld is greatly reduced and transferred to the tank wall and bottom at a certain distance from the weld. The radial and axial residual stresses of the T-shaped weld are reduced to about 158 MPa and 123 MPa respectively, and the circumferential residual stress is reduced to about 416 MPa. At the same time, a partial enlarged view of the weld is output. There is still welding residual tensile stress only at the geometric discontinuity, but the stress value has also been greatly reduced.

[0255] (5) Heat treatment results with an 800 mm heating band

[0256] The heat treatment results show that the residual tensile stress at the weld is greatly reduced and transferred to the tank wall and bottom at a certain distance from the weld. The radial and axial residual stresses of the T-shaped weld are reduced to about 155 MPa and 186 MPa respectively, and the circumferential residual stress is reduced to about 433 MPa. At the same time, a partial enlarged view of the weld is output. There is still welding residual tensile stress only at the geometric discontinuity, but the stress value has also been greatly reduced.

[0257] (6) Heat treatment results with a 1000 mm heating band

[0258] The heat treatment results in a significant reduction in the residual tensile stress at the weld and its transfer to the tank wall and bottom at a certain distance from the weld. The radial and axial residual stresses of the T-joint weld are reduced to about 400 MPa and 286 MPa respectively, and the circumferential residual stress is reduced to about 348 MPa. Meanwhile, a partial enlarged view of the weld is output. There is still welding residual tensile stress only at the geometric discontinuity, but the stress value has also been significantly reduced.

[0259] (7) Heat treatment results of 1200 mm heating band

[0260] The heat treatment results in a significant reduction in the residual tensile stress at the weld and its transfer to the tank wall and bottom at a certain distance from the weld. The radial and axial residual stresses of the T-joint weld are reduced to about 485 MPa and 369 MPa respectively, and the circumferential residual stress is reduced to about 500 MPa. Meanwhile, a partial enlarged view of the weld is output. There is still welding residual tensile stress only at the geometric discontinuity, but the stress value has also been significantly reduced.

[0261] By comparing the residual stresses after heat treatment with different heating band widths, it can be seen that the heat treatment effect is the best when using a 200 mm wide heating band. The local reduction of the circumferential residual stress can reach 50%, and the heat treatment effect is good.

[0262] According to the requirements of NB / T 47014-2011 "Welding Procedure Qualification for Pressure Equipment" and GB / T 30583-2014 "Code for Post-Weld Heat Treatment of Pressure Equipment", through finite element calculation and research on factors such as the holding temperature, cooling rate, and heating band width of the heat treatment, the optimal heat treatment plan for the T-joint weld is determined as follows: the holding temperature is 700 °C, the heating rate is 56 °C / h, and the heating band width is 200 mm. After heat treatment regulation, the circumferential welding residual stress decreases significantly, and the maximum reduction of the circumferential welding residual stress is 48% - 53%.

[0263] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included in the protection scope of the present invention.

[0264] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of various equivalent modifications or replacements, and these modifications or replacements should all be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

[0265] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for regulating the residual stress of the weld of a crude oil storage tank, characterized in that, It includes the following steps: S110. Process a storage tank specimen according to the size and welding process of the crude oil storage tank; S120. Establish a finite element model for the storage tank specimen to simulate the welding process of the storage tank specimen; Determine the boundary conditions by simulating the mechanical property parameters and thermal property parameters of the storage tank specimen during the welding process through finite element simulation; calculate the changes in the real-time welding stress and stress field of the entire storage tank specimen during the welding process through thermo-elasto-plastic analysis, and obtain the distribution law of the welding residual stress along the thickness and height; S130. Conduct residual stress testing on the storage tank specimen to obtain the residual stress test results of the storage tank specimen; S140. Verify the distribution law of the welding residual stress along the thickness and height through the residual stress test results of the storage tank specimen; if the verification is successful, determine the weld with the maximum residual stress as the key position, simulate the heat treatment conditions of the key position through the finite element model, and obtain the optimal heat treatment plan when the residual stress is the lowest under a certain heat treatment condition.

2. The method for regulating the residual stress of the weld seam of the crude oil storage tank according to claim 1, wherein In S110, the storage tank specimen is processed from several materials according to the structure of the crude oil storage tank.

3. The method for regulating the residual stress of the weld seam of the crude oil storage tank according to claim 1, wherein In S120, when establishing a finite element model and performing geometric mesh division on the storage tank specimen, it includes the following steps: (1) Establish a local two-dimensional axisymmetric model according to the size and structure of the storage tank specimen; (2) Conduct mesh division on the local two-dimensional axisymmetric model, set the mesh size of the weld and the heat affected zone to the first density size, set the mesh size far from the weld to the second density size, and use transition meshes for division in the middle part, where the first density size is smaller than the second density size; (3) Adopt two-dimensional four-node tetrahedron reduced integration elements and use the element passivation and activation method to form the weld metal.

4. The method for regulating the residual stress of the weld seam of the crude oil storage tank according to claim 1, wherein In S120, the process of simulating the mechanical property parameters and thermal property parameters of the storage tank specimen during the welding process includes: (1) During the temperature field calculation process, set the absolute zero temperature and the ambient temperature; assign the ambient temperature to the model during welding calculation, assign the preheating temperature to the model during heat treatment calculation, and set the convective heat dissipation and radiative heat dissipation of the model at the same time; (2) Fix both ends of the storage tank specimen according to the actual welding situation to prevent movement and calculate the boundary conditions of the stress field.

5. The method for regulating the residual stress of the weld seam of the crude oil storage tank according to claim 1, wherein In S120, the process of obtaining the distribution law of the welding residual stress along the thickness and height is as follows: Conduct edge plate butt welding simulation, conduct variable wall thickness circumferential welding simulation for the first and second laps of the tank wall, conduct longitudinal welding simulation for the first lap of the tank wall, conduct longitudinal welding simulation for the first lap of the tank wall, conduct variable wall thickness welding simulation for the edge plate and the middle plate of the tank bottom plate, conduct butt weld simulation for the middle plate of the tank bottom plate, conduct T-shaped weld simulation for the tank wall and the tank bottom plate, and obtain the distribution law of the welding residual stress along the thickness and height.

6. The method for regulating the residual stress of the weld seam of the crude oil storage tank according to claim 1, characterized in that, The residual stress test results include the residual stress on the structure surface, the residual stress in the thickness direction, and the residual stress of each cross-section.

7. The method for regulating the residual stress of the weld seam of the crude oil storage tank according to claim 6, characterized in that, The objects for residual stress testing of the crude oil storage tank include edge plate V-shaped butt test plates, wall plate circumferential weld K-shaped butt test plates, wall plate longitudinal weld X-shaped groove butt test plates, middle plate V-shaped butt test plates, edge plate - middle plate V-shaped butt test plates, and large fillet weld T-shaped corner joint test plates.

8. The method for regulating the residual stress of the weld seam of the crude oil storage tank according to claim 7, characterized in that, The edge plate V-shaped butt joint test plate uses indentation method, neutron diffraction and contour method to measure the welding residual stress; the remaining test plates are selected to use the indentation method for measurement.

9. The method for regulating the residual stress of the weld seam of the crude oil storage tank according to claim 1, characterized in that In S140, the heat treatment conditions for simulating the key positions include holding temperature, cooling rate and heating zone width.

10. The method for regulating the residual stress of the weld seam of the crude oil storage tank according to any one of claims 1-9, characterized in that, The key position is the T-shaped weld.

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