Partial heat treatment method for large pressure vessel with uneven thickness structure

Abstract

This invention discloses a method for local heat treatment of unequal thickness structures in large pressure vessels, used for heat treatment of butt welded joints between the cylinder and reinforcing plate. The method includes the following steps: (1) determining the relevant dimensional parameters of the unequal thickness welded joint, including the inner diameter D of the cylinder, the cylinder wall thickness T, and the thickness t of the reinforcing plate. Then, the inner radius R of the cylinder is R = D / 2, the thickness ratio k = t / T, and t > T; (2) determining the local heat treatment heating method; (3) determining the optimal heat treatment process parameters, including the heating band width, holding temperature, holding time, heating rate, and cooling rate; (4) implementing the heat treatment and recording the heat treatment temperature curve. The method provided by this invention can provide optimal heating band width, holding temperature, holding time, heating rate, and cooling rate for local heat treatment of unequal thickness welded joints, effectively reducing inconsistent deformation and uneven stress distribution after local heat treatment of thick and thin plates, and preventing cracking of the unequal thickness welded joint.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology, and specifically to a method for local heat treatment of uneven thickness structures in large pressure vessels. Background Technology

[0002] Large pressure vessels generate significant residual stress during the welding process, making them prone to defects such as cracks in the weld joint area. Post-weld heat treatment is a common method for eliminating residual stress, and it can be divided into overall heat treatment and local heat treatment. Due to the enormous size of large pressure vessels, overall heat treatment is not feasible; therefore, local heat treatment is the primary method for eliminating residual stress, thereby ensuring the structural integrity of the pressure vessel.

[0003] Currently, local heat treatment of welded joints in large pressure vessels is often carried out according to existing post-weld heat treatment specifications. However, these post-weld heat treatment methods are general provisions and do not consider factors such as material and size. Furthermore, local post-weld heat treatment standards vary from country to country, making it difficult to formulate post-weld heat treatment processes. In addition, post-weld heat treatment is influenced by numerous factors and is a complex process. Process parameters such as heating band width, holding temperature, holding time, heating rate, and cooling rate are important control parameters for local post-weld heat treatment and directly affect the quality of heat treatment. Improper local heat treatment not only fails to effectively reduce welding residual stress but also generates additional secondary residual stress. Moreover, after the segmented welding of the cylinder of a large steel pressure vessel is completed, openings need to be made in the cylinder to facilitate material transportation and personnel access. To compensate for the decrease in strength around the openings, reinforcing plates need to be connected for reinforcement. The thickness of the reinforcing plate is generally greater than that of the cylinder wall. Local heat treatment of the weld connecting the reinforcing plate and the cylinder can lead to an asymmetric temperature field distribution due to structural asymmetry, thus affecting the residual stress field and causing variations in deformation and stress conditions between thick and thin plates. Therefore, it is necessary to rationally set the heating band arrangement and heat treatment process parameters. Determining the heating band width and process parameters through a series of heat treatment experiments is time-consuming, costly, and lacks foresight. Therefore, numerical simulation methods can be used to determine the heating band width and process parameters. However, traditional welding and heat treatment simulations, especially in welding temperature field simulations, neglect the influence of other factors such as molten metal on the welding temperature field. This cannot accurately guide the prediction of welding and heat treatment residual stress, resulting in some deviation in the predicted heat treatment process parameters. Consequently, it is impossible to formulate accurate local heat treatment methods for large pressure vessels with unequal thickness welded structures. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for local heat treatment of large pressure vessels with unequal thickness structures. The method employs numerical simulation to determine heat treatment process parameters such as the width of the heating zone, holding temperature, holding time, heating rate, and cooling rate. Furthermore, the simulation process is improved to enhance the accuracy of the welding process and the simulation results of the local heat treatment, thereby guiding the implementation of the local heat treatment.

[0005] The present invention specifically adopts the following technical solution:

[0006] A method for localized heat treatment of uneven thickness structures in large pressure vessels, used for heat treatment of butt welds between the cylinder and reinforcing plates, includes the following steps:

[0007] (1) Determine the relevant dimensional parameters of the unequal thickness welded joint: including the inner diameter D of the cylinder, the wall thickness T of the cylinder, and the thickness t of the reinforcing plate. Then the inner radius R of the cylinder is R = D / 2, the thickness ratio k = t / T, and t > T.

[0008] (2) Determine the heating method for local heat treatment;

[0009] (3) Determine the optimal process parameters for local heat treatment: including heating band width, holding temperature, holding time, heating rate and cooling rate, and the heating band width, holding temperature, holding time, heating rate and cooling rate are obtained through numerical simulation.

[0010] (4) Heat treatment was carried out and the heat treatment temperature curve was recorded.

[0011] Furthermore, in step (2), the local heat treatment heating method is electromagnetic induction heating or resistance heating.

[0012] Furthermore, in step (3), the width of the local heat treatment heating zone obtained through simulation is...

[0013] HB1 is the main heating width, HB2 is the width of the auxiliary heating strip on the side of the reinforcing plate, and the main heating strip is arranged with equal width on both sides of the weld.

[0014] Furthermore, in step (3), the specific steps for determining the optimal process parameters for local heat treatment using simulation methods are as follows:

[0015] (31) Establishment of model of unequal thickness welded joint

[0016] Determine the relevant dimensional parameters of the unequal thickness welded joint: including the inner diameter D of the cylinder, the wall thickness T of the cylinder, and the thickness t of the reinforcing plate. Then the inner radius R of the cylinder is R = D / 2, the thickness ratio k = t / T, and t > T. Based on the above dimensional parameters, establish a two-dimensional axisymmetric model, i.e., the unequal thickness welded joint model, and perform mesh generation and assign material properties.

[0017] (32) Simulation of welding temperature field

[0018] Based on the unequal thickness welded joint model established in step (31), a welding moving heat source model and boundary conditions are applied, and the heat transfer coefficient is set to simulate the welding temperature field. The simulation calculation process of the welding temperature field is divided into two stages, namely:

[0019] The first stage is the time period from the start point when the welding heat source acts on the workpiece to the end of the welding process. This first stage requires the establishment of a numerical calculation model based on fluid dynamics methods, and the influence of the flow behavior of the molten pool on the temperature distribution. The heating section of the welding process is calculated to obtain the temperature field data at each time step in this first stage.

[0020] The second stage is the time period from the end of welding to the cooling of the weldment to room temperature. This second stage requires the establishment of a numerical calculation model based on the finite element method, and the acquisition of temperature field data at each time step in the second stage based on the temperature distribution at the last time step calculated in the first stage.

[0021] (33) Simulation of welding stress field

[0022] In the simulation of welding stress field, the FEM method was used to calculate the stress field in the first and second stages respectively;

[0023] (34) Local heat treatment simulation

[0024] Based on the stress field obtained in step (33) for the second stage of welding, the local heat treatment temperature field is simulated on the unequal thickness welded joint model according to the local heat treatment process parameters, and different thickness ratios and heating band widths are set. The temperature field and stress field with different equal thickness ratios and different heating band widths are calculated by restart analysis. The stress field obtained in this step is the result of the residual stress distribution after local heat treatment.

[0025] (35) Determine the optimal process parameters for local heat treatment

[0026] The effects of thickness ratio, heating band width, holding temperature, holding time, heating rate and cooling rate on the elimination of residual stress after welding were analyzed in step (34) of the local heat treatment simulation process, and the optimal process parameters were determined in the local heat treatment process.

[0027] Furthermore, in step (33), during the process of calculating the stress fields of the first stage and the second stage using the FEM method, it is necessary to load the temperature field data obtained in the first stage and the second stage into the FEM calculation model, and use the temperature field data as a predefined field to perform thermal stress coupling calculation.

[0028] Furthermore, the local heat treatment temperature field simulation process in step (34) includes a heating stage, a heat holding stage, and a cooling stage;

[0029] For the heating and holding stages, a heating band is covered outside the unequal thickness welded joint model, and a forced convection heat transfer coefficient is set in the area covered by the heating band. The temperature is increased and held according to the amplitude of the local heat treatment temperature curve to obtain the temperature field data of the heating and holding stages.

[0030] For the cooling phase, the thermal insulation protection is removed, and natural cooling is carried out to obtain temperature field data for the cooling phase.

[0031] Furthermore, in the local heat treatment temperature field simulation of step (34), when different heating band widths are set, the optimal heating band width for local heat treatment of the weld joint with equal thickness is determined. The main heating band for local heat treatment of unequal-thickness weld joints is arranged with equal widths on both sides of the weld, and an auxiliary heating band is set on one side of the reinforcing plate. The total width of the heating band for local heat treatment of unequal-thickness weld joints is the sum of the widths of the main heating band and the auxiliary heating band. The total width of the heating band for local heat treatment of unequal-thickness weld joints can be adjusted by adjusting the width of the auxiliary heating band. This invention has the following beneficial effects:

[0032] (1) The local heat treatment method for the uneven thickness structure of large pressure vessel provided by the present invention can provide the best process parameters such as heating band width, heat preservation temperature, heat preservation time, heating rate and cooling rate for local heat treatment of uneven thickness welded joints, effectively reducing the uneven deformation and uneven stress distribution after local heat treatment of thick and thin plates, and preventing cracking of uneven thickness welded joints.

[0033] (2) The local heat treatment method for large pressure vessel unequal thickness structure provided by the present invention fully considers the influence of complex molten pool flow on temperature field and residual stress during the simulation process, divides the welding-heat treatment temperature field simulation into two stages, realizes accurate prediction of welding residual stress, and has certain guiding significance for correctly formulating accurate heat treatment process for large pressure vessel unequal thickness welded structure. Attached Figure Description

[0034] The present invention will be further described below with reference to the accompanying drawings:

[0035] Figure 1 A flowchart illustrating the calculation process for the first stage of welding in determining the optimal heating band width in an embodiment of the present invention;

[0036] Figure 2 This is a schematic diagram of the heating band arrangement during the local heat treatment simulation of a welded joint model with unequal thickness.

[0037] Figure 3Comparison of simulated and measured welding temperature field curves for unequal thickness welded joints in Example 1;

[0038] Figure 4 The simulated and measured curves of the heat treatment stress field of the unequal thickness welded joint in Example 1 are compared. Detailed Implementation

[0039] To make the advantages and technical solutions of the present invention clearer and more explicit, the present invention will be described in detail below with reference to specific embodiments and accompanying drawings.

[0040] Reference Figure 1-4 This embodiment takes an unequal-thickness welded joint obtained by butt welding a reinforcing plate to the cylinder of a large pressure vessel through-sleeve as an example. First, the optimal heating band width, holding temperature, holding time, heating rate, and cooling rate are determined through multi-physics field coupling simulation of welding and heat treatment for the local heat treatment of the unequal-thickness welded joint. Then, local heat treatment is implemented based on the determined optimal heat treatment parameters. The reinforcing plate and the cylinder of this unequal-thickness welded joint are made of the same material, SA738Gr.B pressure vessel steel. The cylinder and the reinforcing plate are connected by a butt full-penetration weld. The local heat treatment area after welding is the weld connecting the reinforcing plate and the cylinder.

[0041] This embodiment uses the determination of the optimal heating band width through simulation as an example to illustrate the simulation method of the present invention. The steps for determining the optimal heating band width for local heat treatment of unequal thickness welded joints through simulation are as follows:

[0042] (a) Modeling of unequal thickness welded joint

[0043] A two-dimensional axisymmetric model, namely the unequal thickness welded joint model, is established based on the relevant dimensional parameters of the unequal thickness welded joint. The model is then meshed and assigned material properties. In this model, the inner diameter D of the cylinder of the unequal thickness welded joint is set to 43000mm, the thickness T is 52mm, and the thickness t of the reinforcing plate is 100mm. Both the reinforcing plate and the cylinder are made of SA738Gr.B material.

[0044] (b) Simulation of welding temperature field

[0045] Based on the unequal thickness welded joint model established in step (a), the convective heat transfer coefficient is set to 10 W / (m²). 2 K), due to the different thicknesses of the cylinder and the reinforcing plate, the thermal conductivity coefficients of the weld and its two sides differ during welding and heat treatment. Furthermore, the material exhibits anisotropy. Therefore, different thermal conductivity coefficients are pre-set in the x, y, and z directions. These coefficients change continuously with temperature. A welding moving heat source model and boundary conditions are applied to simulate the welding temperature field. The simulation calculation process is divided into two stages:

[0046] The first stage is the time period from the start point when the welding heat source acts on the workpiece to the end of the welding process. In this first stage, the workpiece metal is heated and melted to form a molten pool. A numerical calculation model is established using the fluid dynamics (CFD) method, and the influence of the flow behavior of the molten pool on the temperature distribution is considered. The heating section of the welding process is calculated to obtain the temperature field data at each time step in this first stage. The finite volume method (FVM) is used in the above calculation process.

[0047] Specifically, in the process of calculating the welding temperature field in the first stage using the fluid dynamics (CFD) method, the coupling effects of buoyancy force, recoil pressure caused by liquid metal evaporation and surface tension, liquid static pressure and shear force are comprehensively considered, that is, the flow field changes in the first stage of welding (i.e. the weldment metal is heated and melted to form the molten pool morphology).

[0048] In the above calculation of flow field and temperature field, the governing equations that need to be satisfied are the mass conservation equation, the momentum conservation equation, and the energy conservation equation.

[0049] The mass conservation equation is:

[0050]

[0051] Where D is the divergence, and u, v, and w are the velocity components in the x, y, and z directions, respectively;

[0052] The momentum conservation equation is:

[0053]

[0054]

[0055]

[0056] Where g is the acceleration due to gravity, ρ is the density, p is the pressure, and μ is the kinematic viscosity of the fluid;

[0057] The energy conservation equation is:

[0058]

[0059] Where Q is the internal heat source of the fluid, λ is the thermal conductivity of the fluid, and c is the specific heat capacity.

[0060] In addition, the steam back pressure P during the welding process r =A·P s Surface tension P σ =κ·γ, Marangoni shear force γ(T)=γ m +δ(TT m ), thermal buoyancy S=ρgβ(T-T0), hydrostatic pressure P of the liquidl The driving forces such as ρgz are treated as source terms and added to the above control equations through UDF subroutines.

[0061] In addition, the variables of the above-mentioned control equations are coupled with each other during the solution process. In order to reduce the number of iteration steps and shorten the computation time, the PISO algorithm can be used to solve the discrete equation system of the control equations.

[0062] The second stage is the time period from the end of welding at the heat source to the workpiece cooling to room temperature. This second stage requires establishing a numerical calculation model based on the finite element method (FEM), and obtaining temperature field data for each time step in this second stage based on the temperature distribution calculated at the last time step in the first stage. In this second stage, as the temperature decreases, the weld pool resolidifies, and there is no flow state in the weld metal. Therefore, after the weld pool solidifies, the influence of flow on heat transfer is no longer considered, meaning there is no need to solve the temperature field using CFD methods.

[0063] (c) Simulation of welding stress field

[0064] In the welding stress field simulation process, in order to avoid rigid displacement of the model, radial and axial displacement constraints were applied to the upper right corner of the model, radial displacement constraints were applied to the lower left corner of the model, and axial displacement constraints were applied to the lower right corner of the model, so that the unequal thickness welded joint model can expand freely in the radial direction during the welding simulation; then the FEM method was used to calculate the stress field of the first stage and the second stage respectively.

[0065] In the process of calculating the welding stress field in the first stage using the FEM method, the temperature field data of the first stage needs to be loaded into the FEM calculation model. The temperature field data is used as the predefined field for the corresponding analysis step calculation, and thermal stress coupling calculation is performed. In the process of calculating the welding stress field in the second stage using the FEM method, the temperature field data of the second stage needs to be loaded into the FEM calculation model. The temperature field data is used as the predefined field for the corresponding analysis step calculation, and thermal stress coupling calculation is performed based on the stress field obtained in the first stage.

[0066] In addition, please refer to the specific details. Figure 1Because the first stage uses the finite volume method (FVM) to calculate the temperature field using CFD, while the stress and deformation calculation uses the finite element method (FEM), these two methods differ in the partitioning of the control volume, leading to inconsistencies in their respective computational data storage locations. Therefore, it is necessary to couple the CFD and FEM methods. Thus, MATLAB employs a binary search algorithm to first determine the center position of the control volume in the FVM, and then determine the mesh nodes around the center of the control volume. This achieves data sharing and computational efficiency during the coupled computation process of the CFD and FEM models (i.e., corresponding...). Figure 1 The process involves two steps: outputting and importing the mesh. Specifically, a welding numerical calculation model based on thermal-fluid-structure interaction was established using both CFD and FEM methods, enabling numerical calculation and analysis of welding stress and deformation that considers molten pool flow behavior and weld morphology.

[0067] (d) Simulation of local heat treatment

[0068] Based on the stress field of the second stage of welding obtained in step (c), local heat treatment simulation was performed on the unequal thickness weld joint model according to the local heat treatment process parameters, and different thickness ratios and heating band widths were set. The arrangement of the heating band width is as follows: Figure 2 As shown, the temperature and stress fields with different thickness ratios and different heating band widths are then calculated using restart analysis. The stress field obtained in this step is the result of the residual stress distribution after local heat treatment.

[0069] Specifically, in the local heat treatment simulation of step (d) above, the inner diameter D of the cylinder of the unequal thickness welded structure is first set to 43000 mm, the thickness T to 52 mm, and the thickness t of the reinforcing plate to 100 mm. That is, the thickness ratio of the unequal thickness welded structure is k = 2, and the optimal heating band width for local heat treatment of the equal thickness welded joint is determined. Based on the equal width arrangement on both sides of the weld, the temperature change and residual stress / deformation field distribution characteristics of the unequal thickness welded joint are analyzed, and the width of the heating band is taken as... Approximately 3000 mm. A symmetrical 3000 mm heating band is created centered on the weld seam to apply heat treatment temperature loads. Based on the unequal thickness weld joint model and the welding temperature field, a restart analysis is used to first calculate the heat treatment temperature field. Then, based on the obtained heat treatment temperature field, a restart analysis is used to calculate the heat treatment stress field based on the welding stress field. Specifically, for the heating and holding stages in the heat treatment simulation process, the forced convection heat transfer coefficient for the heating band coverage area is set to 8 × 10⁻⁶. 6 W / (m 2K), heating and holding are performed according to the given heat treatment temperature curve amplitude (i.e., local heat treatment process parameters). Outside the heating zone, natural convection heat transfer is used. During the heating stage, the heating zone needs to be divided into multiple squares of equal area. A surface heat source is set in each square, and the heating radius of each heat source is controlled to achieve full coverage of the heat source in each square area. The analysis step time is controlled to make the heat treatment temperature reach the requirements of the local heat treatment process (615±15℃). For the cooling stage in the heat treatment simulation process, the temperature is reduced to 300℃ according to the heat treatment temperature curve amplitude. Then, the insulation protection is removed, and the cylinder is allowed to cool naturally. The surface convection heat transfer coefficient is set in the same way as in the welding process.

[0070] Through the above simulation of local heat treatment, it was found that when the width of the heating band on both sides of the weld is equal, the deformation of the cylinder and the reinforcing plate is asymmetrical. The cylinder deformation is too large, which causes the thin plate to be constrained and squeezed by the reinforcing plate when the cylinder and the reinforcing plate are naturally cooled. Moreover, the thin plate recovers quickly, resulting in greater axial stress on one side of the cylinder, which is particularly obvious at the weld toe.

[0071] To improve the uneven deformation of the welded joints with unequal thicknesses during the aforementioned localized heat treatment process, and to address the excessive deformation of the cylinder after heat treatment, an auxiliary heating band with a width of HB2 is installed on one side of the reinforcing plate. This provides more heat input to the reinforcing plate, resulting in greater deformation to accommodate the thermal expansion of the cylinder. Based on the dimensions of the unequal thickness welded joint model formed by butt welding the cylinder and the reinforcing plate, the heating band width arrangement is determined: [The heating band width during the aforementioned process is...] As the main heating band width HB1, and with the main heating band width arranged at equal widths on both sides of the weld, the auxiliary heating band width HB2 on one side of the reinforcing plate is set at intervals of 250mm from 250mm to 2250mm, for a total of 9 different heating band widths. The optimal heating band width under this thickness ratio is determined to be 4500mm. Then, different thickness ratios k=1.5 and 2.5 are set respectively, and the optimal heating band widths under different thickness ratios are determined to be 4250mm and 4750mm respectively.

[0072] (e) Determine the width of the local heat treatment heating zone

[0073] Analysis of step (d) reveals the relationship between the thickness ratio, heating band width, and residual stress after local heat treatment during the simulation. This leads to the derivation of the formula for calculating the optimal heating band width during local heat treatment.

[0074]

[0075] Based on the determined width of the heating band for local heat treatment, other process parameters during local heat treatment, namely holding temperature, holding time, heating rate, and cooling rate, are determined using the same simulation method. Specifically, the determined heating band width can be taken as the optimal heating band width. Then, different heat treatment holding temperatures, holding times, heating rates, and cooling rates are adjusted using the same simulation method to conduct simulations. The optimal local heat treatment process parameters are determined based on the obtained residual stress distribution results.

[0076] The optimal process parameters for local heat treatment of unequal thickness welded joints of SA738Gr.B material were determined through simulation (applicable to cylinders with a side plate thickness of 50-100mm):

[0077] Number of heating cycles: n = 3;

[0078] Insulation temperature: First stage insulation temperature 300~400℃, second stage insulation temperature 450~510℃, third stage insulation temperature 615±15℃;

[0079] Insulation time: First stage insulation time t1≥2h, second stage insulation time t2≥2h, third stage insulation time t3≥4h;

[0080] Heating rate: First stage heating rate v1≤25℃ / h, second stage heating rate v2≤20℃ / h, third stage heating rate v3≤10℃ / h;

[0081] Cooling rate: First cool to 300℃ at a rate of ≤55℃ / h, then air cool to room temperature.

[0082] Example 1

[0083] Based on the optimal heating band width, holding temperature, holding time, heating rate, and cooling rate for local heat treatment of unequal thickness welded joints with different thickness ratios determined above, welding and local heat treatment were performed on unequal thickness welded joints. Taking a butt welded joint between a cylinder with an inner diameter D of 43000 mm and a thickness T of 52 mm and a reinforcing plate with a thickness t of 100 mm as an example, the reinforcing plate and the cylinder are made of the same material, SA738Gr.B pressure vessel steel. The cylinder and the reinforcing plate are connected by a butt full penetration weld. The local heat treatment area after welding is the weld connecting the reinforcing plate and the cylinder. The local heat treatment process includes the following steps:

[0084] (1) Determine the relevant dimensional parameters of the unequal thickness welded joint: including the inner diameter of the cylinder D = 43000mm, the cylinder wall thickness T = 52mm, the thickness of the reinforcing plate t = 100mm, then the inner radius of the cylinder R = D / 2 = 21500mm, and the thickness ratio k = t / T ≈ 2.

[0085] (2) Determine the local heat treatment heating method and use ceramic sheet heating;

[0086] (3) Determine the optimal process parameters for local heat treatment:

[0087] Heating band width:

[0088]

[0089] Process parameters such as heat preservation temperature, heat preservation time, heating rate, and cooling rate:

[0090] In this embodiment, the heat treatment process is carried out in three stages: the first stage heats the material to 400°C at a heating rate of 25°C / h and holds it for at least 2 hours; the second stage heats the material to 510°C at a heating rate of 20°C / h and holds it for at least 2 hours; the third stage heats the material to 610°C at a heating rate of 10°C / h, and then the temperature of the heating element is finely adjusted to keep the corresponding thermocouple temperature between 600°C and 630°C, and as close to 615°C as possible. After the reading stabilizes, the holding time begins and is at least 4 hours. During cooling, the material is first cooled to 300°C at a rate of 50°C / h, and then the power to the heat treatment equipment is turned off to allow it to cool naturally to room temperature.

[0091] (4) Heat treatment was carried out and the heat treatment temperature curve was recorded.

[0092] In the welding process of the aforementioned unequal thickness welded joint, thermocouples are used to measure the welding temperature. The welding temperature field simulation method described in step (b) is used to simulate the welding process according to the same welding process parameters, obtaining the welding temperature field. The measured and simulated temperature change curves of the first stage of welding are then obtained. These curves are then simulated using the traditional welding simulation method with the same welding process parameters to obtain the welding temperature field, which is then compared. Figure 3 As shown. From Figure 3 As can be seen, the welding temperature field data simulated by the method of the present invention is closer to the measured values.

[0093] After the heat treatment of the aforementioned unequal thickness welded joint is completed, residual stress is detected using the indentation method. The local heat treatment stress field is simulated using the method described in step (d) above to obtain the residual stress distribution after local heat treatment. This is then simulated using the same heat treatment process and conventional heat treatment methods to obtain the local heat treatment stress field, and the results are compared. Figure 4 As shown. From Figure 4 As can be seen, the residual stress simulated by the method of the present invention after local heat treatment is closer to the measured value, and can more accurately reflect the residual stress elimination effect.

[0094] Additionally, refer to Figure 4Before and after heat treatment, the axial residual stress of the above-mentioned unequal thickness welded joints was basically eliminated and evenly distributed along the center of the weld. This also verifies the rationality of the selection of the optimal heating band width for welded joints with different thickness ratios and the rationality of the selection of heat treatment process parameters. It effectively reduces the situation of inconsistent deformation and uneven stress distribution after local heat treatment of thick and thin plates, and prevents cracking of unequal thickness welded joints.

[0095] The parts not mentioned above can be achieved by drawing on existing technologies.

[0096] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for localized heat treatment of uneven thickness structures in large pressure vessels, used for heat treatment of butt welds between the cylinder and reinforcing plates, characterized in that... Includes the following steps: (1) Determine the relevant dimensional parameters of the unequal thickness welded joint: including the inner diameter D of the cylinder, the wall thickness T of the cylinder, and the thickness t of the reinforcing plate. Then the inner radius R of the cylinder is R=D / 2, the thickness ratio k=t / T, and t>T; (2) Determine the heating method for local heat treatment; (3) Determine the optimal process parameters for local heat treatment: including heating band width, holding temperature, holding time, heating rate and cooling rate, and the heating band width, holding temperature, holding time, heating rate and cooling rate are obtained through numerical simulation; (4) Perform heat treatment and record the heat treatment temperature curve; In step (3), the specific steps for determining the optimal process parameters for local heat treatment using numerical simulation are as follows: (31) Establishment of model of unequal thickness welded joint Based on the relevant dimensional parameters of the unequal thickness welded joint determined in step (1), a two-dimensional axisymmetric model is established, namely the unequal thickness welded joint model, and meshing is performed and material properties are assigned. (32) Simulation of welding temperature field Based on the unequal thickness welded joint model established in step (31), a welding moving heat source model and boundary conditions are applied, and the heat transfer coefficient is set to simulate the welding temperature field. The simulation calculation process of the welding temperature field is divided into two stages, namely: The first stage is the time period from the start point when the welding heat source acts on the workpiece to the end of the welding process. This first stage requires the establishment of a numerical calculation model based on fluid dynamics methods, and the influence of the flow behavior of the molten pool on the temperature distribution. The heating section of the welding process is calculated to obtain the temperature field data at each time step in this first stage. The second stage is the time period from the end of welding to the cooling of the weldment to room temperature. This second stage requires the establishment of a numerical calculation model based on the finite element method, and the acquisition of temperature field data at each time step in the second stage based on the temperature distribution at the last time step calculated in the first stage. (33) Simulation of welding stress field In the simulation of welding stress field, the FEM method was used to calculate the stress field in the first and second stages respectively; (34) Simulation of local heat treatment Based on the stress field obtained in step (33) for the second stage of welding, the local heat treatment temperature field is simulated on the unequal thickness structure model according to the local heat treatment process parameters. Different thickness ratios, heating band widths, holding temperatures, holding times, heating rates and cooling rates are set. The temperature field and stress field under different equal thickness ratios, different heating band widths and different holding temperatures, holding times, heating rates and cooling rates are calculated using restart analysis. The stress field obtained in this step is the result of the residual stress distribution after local heat treatment. (35) Determine the optimal process parameters for local heat treatment Analyze the effects of thickness ratio, heating band width, holding temperature, holding time, heating rate and cooling rate on the elimination of residual stress after welding during the local heat treatment simulation process in step (34), and determine the optimal process parameters during the local heat treatment process. In step (33), during the process of calculating the stress field of the first stage and the second stage using the FEM method, the temperature field data obtained in the first stage and the second stage need to be loaded into the FEM calculation model respectively, and the temperature field data is used as a predefined field for thermal stress coupling calculation. The local heat treatment temperature field simulation process in step (34) includes a heating stage, a heat preservation stage and a cooling stage; For the heating and holding stages, a heating band is covered outside the unequal thickness welded joint model, and a forced convection heat transfer coefficient is set in the area covered by the heating band. The temperature is increased and held according to the amplitude of the local heat treatment temperature curve to obtain the temperature field data of the heating and holding stages. For the cooling stage, the temperature is first cooled to a certain temperature according to the amplitude of the local heat treatment temperature curve, and then the thermal insulation protection is removed and natural cooling is carried out to obtain the temperature field data of the cooling stage. In the local heat treatment temperature field simulation of step (34), when different heating band widths are set, the optimal heating band width for local heat treatment of equal-thickness welded joints is determined. The main heating band for local heat treatment of unequal-thickness welded joints is arranged with equal width on both sides of the weld, and an auxiliary heating band is set on one side of the reinforcing plate. The total width of the heating band for local heat treatment of unequal-thickness welded joints is the sum of the width of the main heating band and the width of the auxiliary heating band. The total width of the heating band for local heat treatment of unequal-thickness welded joints can be adjusted by adjusting the width of the auxiliary heating band.

2. The method for local heat treatment of a large pressure vessel with unequal thickness structure according to claim 1, characterized in that, In step (2), the local heat treatment heating method is electromagnetic induction heating or resistance heating.

3. The method for local heat treatment of a large pressure vessel with unequal thickness structure according to claim 1, characterized in that, In step (3), the width of the local heat treatment heating band for SA738Gr.B pressure vessel steel obtained through simulation is: ; HB1 is the main heating width, HB2 is the width of the auxiliary heating strip on the side of the reinforcing plate, and the main heating strip is arranged with equal width on both sides of the weld.

Images