A method for predicting a reheat crack dangerous zone of a heat treatment

Abstract

The application discloses a heat treatment reheat crack dangerous area prediction method and belongs to the technical field of reheat crack numerical simulation. The method comprises the following steps: a heat treatment finite element model of a component to be analyzed is established, a heat treatment temperature field numerical simulation is carried out, and a heat treatment temperature field is acquired; the heat treatment temperature field is taken as an initial condition, a damage activation condition and a temperature correction creep damage model are simultaneously set, and a damage variable is introduced through a user subroutine; then, a heat treatment stress field numerical simulation is carried out, when the heat treatment damage activation condition is met, the temperature correction creep damage model is started, creep strain increments and damage increments are calculated according to time increments, and a cumulative damage amount and a damage field are acquired; a damage field distribution cloud picture is analyzed, and a heat treatment reheat crack dangerous area is determined. The application can simulate damage accumulation behaviors in the heat treatment heating and holding stages, quantitatively give damage amount distribution, and effectively identify a reheat crack dangerous area in the heat treatment process.

Description

Technical Field

[0001] This invention relates to the field of numerical simulation technology for reheat cracking, and specifically to a method for predicting the danger zone of heat treatment reheat cracking. Background Technology

[0002] Reheat cracking is a type of delayed cracking that easily occurs in welded joints of high-temperature pressure-bearing components during post-weld heat treatment or reheating. This type of cracking is usually closely related to stress relaxation, decreased creep deformation capacity, localized stress concentration, and accumulation of grain boundary damage at high temperatures. For components with high restraint and residual stress, significant ductility depletion may occur in localized areas during heat treatment heating and holding, ultimately inducing reheat cracking. Existing reheat crack prediction methods mainly include the following categories: (1) Empirical criterion method, that is, judging the sensitivity to reheat cracking based on material composition, heat treatment temperature range, hardness or empirical spectrum; this method is convenient to use, but it is difficult to analyze the influence of specific component geometry, restraint state and heat treatment process on the initiation and propagation of reheat cracks, resulting in a low accuracy rate of reheat crack danger zone identification. (2) Static analysis method based on final stress. This method only compares the stress level after heat treatment or at a certain moment. It lacks the characterization of the gradual accumulation of damage throughout the heat treatment process and is difficult to accurately identify the actual cracking danger zone. (3) Experimental analysis method, that is, to determine whether reheat cracks occur by cracking test of heat-treated sample, metallographic observation or fracture analysis; however, this method is costly and time-consuming, and it is difficult to quickly compare different process parameters and structural schemes in the design stage.

[0003] The above three types of reheat crack prediction methods have problems such as low accuracy in identifying reheat crack hazard zones or high experimental costs and long cycles, and are not suitable for optimizing structural or process parameters in the design stage of high-temperature pressure-bearing components. Summary of the Invention

[0004] To address the technical problems existing in the prior art, this invention provides a method for predicting the reheat cracking hazard zone during heat treatment. This method can simulate the damage accumulation behavior during the heat treatment heating and holding stages and quantitatively provide the damage distribution, thereby effectively identifying the reheat cracking hazard zone during heat treatment.

[0005] The technical solution adopted in this invention is as follows: A method for predicting the critical zone of heat treatment reheat cracking includes the following steps: S1. Based on the structural and material parameters of the component to be analyzed, establish a finite element model for heat treatment; S2. Apply heat treatment temperature-time boundary conditions to the heat treatment finite element model established in step S1, perform numerical simulation of the heat treatment temperature field, and obtain the temperature field distribution cloud map during the heat treatment process. S3. Apply load constraints to the heat treatment finite element model established in step S1, and introduce the heat treatment temperature field obtained in step S2. At the same time, establish a temperature-corrected creep damage model and set the heat treatment damage activation condition. Then, perform a numerical simulation of the heat treatment stress field of the component to be analyzed. When the heat treatment damage activation condition is met, start the temperature-corrected creep damage model, calculate the creep strain increment and damage increment according to the time increment, and obtain the cumulative damage amount and damage field distribution cloud map. S4. Analyze the damage field distribution cloud map obtained in step S3 to determine the danger zone of heat treatment reheat cracking.

[0006] Furthermore, in step S1, the component to be analyzed is a high-temperature pressure-bearing component with welded joints, and the component to be analyzed has residual welding stress.

[0007] Furthermore, in step S3, the load constraint is applied in the form of a displacement constraint and used to simulate the stress relaxation behavior of the component to be analyzed under structural constraints during heat treatment.

[0008] Furthermore, the heat treatment damage activation condition set in step S3 is as follows: Current temperature > preset activation temperature threshold.

[0009] Furthermore, the temperature-corrected creep damage model set in step S3 is as follows: ; in: ; ; In the formula, This is the equivalent creep strain increment. The damage evolution coefficient is... The stress sensitivity index, For equivalent stress, This is a stress state correction term. Incrementing to the current time. For the current time, This is a historically adjusted index. Temperature is the activation parameter. The gas constant is The current temperature; For damage drivers associated with cumulative damage, This represents the cumulative damage amount.

[0010] Furthermore, the damage increment in step S3 is obtained based on the equivalent creep strain increment and critical ductility, that is: ; ; in: ; In the formula, For the increase in damage, For the first Accumulated damage over time For the first Accumulated damage over time This is the equivalent creep strain increment. This represents the critical fracture strain under the current stress state. It is the critical ductility coefficient. It is a stress triaxiality.

[0011] Furthermore, the method for determining the danger zone of heat treatment reheat cracking in step S4 is as follows: Set a cumulative damage threshold, and use the area in the damage field distribution cloud map obtained in step S3 where the cumulative damage reaches the cumulative damage threshold as the dangerous zone for heat treatment reheat cracking.

[0012] Furthermore, it also includes determining the first cracking zone based on the damage field distribution cloud map obtained in step S3, specifically: the area where the maximum cumulative damage value is located in the damage field distribution cloud map obtained in step S3 is taken as the first cracking zone.

[0013] Furthermore, it also includes determining the potential crack initiation path based on the damage field distribution cloud map obtained in step S3, specifically: performing connectivity identification on the damage field distribution cloud map obtained in step S3 to determine the potential crack initiation path.

[0014] The beneficial effects of this invention are as follows: This invention provides a method for predicting the reheat cracking hazard zone during heat treatment. It can simulate the damage accumulation behavior during the heating and holding stages of heat treatment and quantitatively provide the damage distribution. This allows for the effective identification of the reheat cracking hazard zone during heat treatment, i.e., the potential initiation location of reheat cracks. It can also analyze the propagation path of reheat cracks. The method of this invention can achieve quantitative assessment and risk prediction of reheat cracks during heat treatment and has good engineering application value. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a flowchart of the prediction method of the present invention; Figure 2 This is a stress field distribution cloud map of a C-shaped ring structure obtained through numerical simulation of heat treatment in an embodiment of the present invention; Figure 3 This is a strain field distribution cloud map of a C-shaped ring structure obtained through numerical simulation of heat treatment in an embodiment of the present invention; Figure 4 This is a damage field distribution cloud map of the C-shaped ring structure obtained through numerical simulation of heat treatment in an embodiment of the present invention; Figure 5 This is a temperature variation curve at the root of the U-shaped notch of the C-ring structure in this embodiment of the invention during the numerical simulation of heat treatment; Figure 6 This is a curve showing the stress variation at the root of the U-shaped notch in the C-ring structure of this invention during the numerical simulation of heat treatment. Figure 7 This is the curve showing the change of equivalent creep strain at the root of the U-shaped notch of the C-ring structure in the embodiment of the present invention during the numerical simulation of heat treatment; Figure 8 The curve showing the change of cumulative damage at the root of the U-shaped notch of the C-ring structure in the embodiment of the present invention during the numerical simulation of heat treatment; Figure 9 This is a damage field distribution cloud map of the damage concentration area at the root of the U-shaped notch in the C-shaped ring structure in an embodiment of the present invention. Detailed Implementation

[0017] This invention provides a method for predicting the critical zone of reheat cracking after heat treatment. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0018] Reference Figure 1 This embodiment provides a method for predicting the critical zone of heat treatment reheat cracking, and the specific steps are as follows: S1. Based on the structural and material parameters of the component to be analyzed, establish a finite element model for heat treatment; S2. Apply heat treatment temperature-time boundary conditions to the heat treatment finite element model established in step S1, perform numerical simulation of the heat treatment temperature field, and obtain the temperature field distribution cloud map during the heat treatment process. S3. Apply load constraints to the heat treatment finite element model established in step S1 and introduce the heat treatment temperature field obtained in step S2. At the same time, establish a temperature-corrected creep damage model and set the heat treatment damage activation conditions. Introduce damage variables in the numerical simulation process through a user subroutine. Then, perform a numerical simulation of the heat treatment stress field of the component to be analyzed. When the heat treatment damage activation conditions are met, start the temperature-corrected creep damage model, calculate the creep strain increment and damage increment according to the time increment, and obtain the cumulative damage amount and damage field distribution cloud map. S4. Analyze the damage field distribution cloud map obtained in step S3 to determine the danger zone of heat treatment reheat cracking.

[0019] Specifically, in step S1 above, the component to be analyzed is a high-temperature pressure-bearing component with a welded joint (including a heat-affected zone), and the component to be analyzed has residual welding stress.

[0020] Specifically, in step S3 above, the load constraint is applied in the form of a displacement constraint, and the displacement is kept constant during the numerical simulation process to simulate the stress relaxation behavior of the component to be analyzed under structural constraints during heat treatment.

[0021] Specifically, the heat treatment damage activation condition set in step S3 above is as follows: The current analysis step number is greater than or equal to the preset heat treatment analysis step or the current temperature is greater than the preset activation temperature threshold.

[0022] In the above technical solution, setting heat treatment damage activation conditions is to avoid introducing invalid damage calculations in non-heat treatment stages. When the activation conditions are not met, only the existing state variables are maintained and no damage increment is updated. When the activation conditions are met, the temperature-corrected creep damage model is started, and the creep strain increment and damage increment are calculated according to the time increment.

[0023] Specifically, the temperature-corrected creep damage model set in step S3 above is as follows: ; in: ; ; In the formula, This is the equivalent creep strain increment. , , These are the material fitting parameters obtained through high-temperature creep experiments. The damage evolution coefficient is... The stress sensitivity index, This is a historically adjusted index. For equivalent stress, This is a stress state correction term. Incrementing to the current time. For the current time, Temperature is the activation parameter. The gas constant is The current temperature; For damage drivers associated with cumulative damage, This represents the cumulative damage amount.

[0024] Specifically, the damage increment in S3 above is obtained based on the equivalent creep strain increment and critical ductility, that is: ; ; in: ; In the formula, For the increase in damage, For the first Accumulated damage over time For the first Accumulated damage over time This is the equivalent creep strain increment. This represents the critical fracture strain under the current stress state. It is the critical ductility coefficient, obtained through calibration and fitting via high-temperature creep experiments. For stress triaxiality, , For average stress, This is the equivalent stress, i.e., von Mises stress.

[0025] Specifically, the method for determining the risk zone of heat treatment reheat cracking in step S4 above is as follows: Set a cumulative damage threshold, and use the area in the damage field distribution cloud map obtained in step S3 where the cumulative damage reaches the cumulative damage threshold as the dangerous zone for heat treatment reheat cracking.

[0026] In addition, based on the heat treatment reheat cracking danger zone determined in step S4, the area where the maximum cumulative damage is located in the heat treatment reheat cracking danger zone can also be taken as the first cracking zone.

[0027] Furthermore, based on the heat treatment reheat cracking hazard zone determined in step S4, connectivity identification can be performed on the hazard zone in the damage field distribution cloud map obtained in step S3 to determine potential crack initiation paths. Specifically, based on the damage field distribution cloud map, the regions where the cumulative damage has reached the cumulative damage threshold and the regions that are about to reach the cumulative damage threshold are determined. These regions are continuous, and the direction from the region where the cumulative damage has reached the cumulative damage threshold to the region that is about to reach the cumulative damage threshold is the potential crack initiation path, along which the crack will propagate.

[0028] To better understand the above-mentioned method for predicting the risk zone of reheat cracking after heat treatment, a C-ring structure with a U-shaped notch is used as the component to be analyzed. In this embodiment, the C-ring structure is made of 2.25Cr1Mo0.25V alloy, with an outer diameter of 25mm and a wall thickness of 3mm. The notch is U-shaped and located in the middle region of the outer wall of the C-ring structure, with a depth of 0.76mm and an opening angle of 45°. Furthermore, in this embodiment, the values ​​of each parameter are as follows: , , , , .

[0029] In this embodiment, a heat treatment finite element model is first established in Abaqus finite element simulation software using the dimensional parameters of the aforementioned C-ring structure. In this embodiment, the U-shaped notch of the C-ring structure is used to simulate welded joints or stress concentration areas. This structure can effectively characterize the stress concentration behavior of typical reheat crack-sensitive areas (welds and heat-affected zones) in engineering.

[0030] Based on the established finite element model of heat treatment, a thermo-mechanical coupling analysis method is used to calculate the temperature and stress fields. Specifically, the temperature field calculation is performed first, requiring the definition of material properties (such as thermal conductivity and specific heat capacity), and the application of heat treatment temperature-time boundary conditions, i.e., applying the heat treatment process (specifically: first heating from room temperature to 675℃ at a rate of 50℃ / h, then holding at 675℃ for 8 hours, followed by cooling to 400℃ at a rate of 50℃ / h, and then cooling back to room temperature at a rate of 40℃ / h), and creating a thermal analysis step; then, a numerical simulation of the heat treatment temperature field is performed to obtain the temperature field distribution cloud map during the heat treatment process; then, the temperature field results are used as initial conditions to apply a 0.5mm displacement constraint to both ends of the C-shaped ring structure while keeping the displacements constant, and the structural analysis job is submitted to calculate thermal stress and deformation, obtaining the stress field distribution cloud map during the heat treatment process, such as... Figure 2As shown. In addition, during the stress field calculation, a damage variable is introduced into the numerical simulation process via a user subroutine. Simultaneously, the heat treatment damage activation threshold is set to 450℃. When the temperature exceeds 450℃, the temperature-corrected creep damage model is activated, calculating the creep strain increment and damage increment over time, and calculating the cumulative damage. Finally, equivalent creep strain distribution cloud maps and damage field distribution cloud maps are obtained, as shown below. Figure 3 and Figure 4 As shown.

[0031] In addition, this embodiment also extracts the variation trends of temperature, von Mises stress, equivalent creep strain, and cumulative damage at a point at the root of the U-shaped notch of the C-ring structure during the numerical simulation of heat treatment, as shown below. Figure 5-8 As shown, the correlation between cumulative damage during heat treatment and temperature, stress, and equivalent creep strain is analyzed.

[0032] Regarding the above Figure 2 Analysis revealed significant stress concentration at the root of the U-shaped notch in the C-ring structure, with a maximum equivalent stress of approximately 437 MPa. This indicates that during heat treatment, the coupling effect of temperature gradient and structural constraints creates a high-stress state in the notch region, which is a potential initiation site for reheat cracks.

[0033] Regarding the above Figure 3 Analysis of the figure reveals the mechanical mechanism of reheat crack initiation. During the stress-relieving heat treatment process, stress relaxation occurs inside the component, accompanied by the accumulation of creep strain. Due to the strong stress concentration at the notch root, the creep deformation rate in this region is much higher than that of the surrounding matrix material. When the accumulated equivalent creep strain (CEEQ) in this local area exceeds the creep plastic exhaustion limit of the component at high temperature, it will trigger the initiation of microcracks, which is a typical characteristic of reheat crack formation.

[0034] Regarding the above Figure 4 Analysis revealed that the areas in the damage field distribution cloud map where the cumulative damage reached the cumulative damage threshold were mainly concentrated at the root of the U-shaped notch, and the location of the damage concentration was highly consistent with the location of the stress concentration. This area is a reheat cracking danger zone and cracking occurs preferentially.

[0035] Regarding the above Figure 5-8Analysis revealed the damage evolution process of the component under thermo-mechanical coupling. The initiation, propagation, and accumulation of equivalent creep strain showed a significant positive correlation, while the von Mises stress showed a negative correlation. Damage accumulation primarily occurred during the heating stage. Specifically, during the heating stage, the component was mainly heated, but the temperature did not reach the damage threshold, and damage had not yet initiated. After reaching the damage threshold, as the equivalent creep strain increased sharply, micro-defects within the component rapidly expanded, leading to a rapid increase in damage value and simultaneously inducing material softening, manifested as a subsequent decrease in von Mises stress. In the subsequent stage, as the equivalent creep strain tended to stabilize, the damage growth rate slowed down and eventually stabilized at a certain level, at which point the stress also decreased to its minimum.

[0036] In addition, this embodiment also... Figure 4 The damage field distribution cloud map of the concentrated damage region at the root of the U-shaped notch in the C-ring structure was extracted and analyzed, such as... Figure 9 As shown, from Figure 9 As can be seen, the damage extends inward from the tip of the notch, exhibiting typical characteristics of reheat crack initiation.

[0037] It should be noted that any parts not mentioned in this invention can be achieved by using or referencing existing technologies.

[0038] 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 predicting the critical zone of reheat cracking after heat treatment, characterized in that, Including the following steps: S1. Based on the structural and material parameters of the component to be analyzed, establish a finite element model for heat treatment; S2. Apply heat treatment temperature-time boundary conditions to the heat treatment finite element model established in step S1, perform numerical simulation of the heat treatment temperature field, and obtain the temperature field distribution cloud map during the heat treatment process. S3. Apply load constraints to the heat treatment finite element model established in step S1, and introduce the heat treatment temperature field obtained in step S2. At the same time, establish a temperature-corrected creep damage model and set the heat treatment damage activation condition. Then, perform a numerical simulation of the heat treatment stress field of the component to be analyzed. When the heat treatment damage activation condition is met, start the temperature-corrected creep damage model, calculate the creep strain increment and damage increment according to the time increment, and obtain the cumulative damage amount and damage field distribution cloud map. S4. Analyze the damage field distribution cloud map obtained in step S3 to determine the dangerous zone of heat treatment reheat crack. The temperature-corrected creep damage model set in step S3 is as follows: ； in: ； ； In the formula, This is the equivalent creep strain increment. The damage evolution coefficient, The stress sensitivity index, For equivalent stress, This is a stress state correction term. Incrementing to the current time. For the current time, This is a historically adjusted index. Temperature is the activation parameter. The gas constant is The current temperature; For damage drivers associated with cumulative damage, This represents the cumulative damage amount; In step S3, the damage increment is obtained based on the equivalent creep strain increment and critical ductility, that is: ； ； in: ； In the formula, For the increase in damage, For the first Accumulated damage over time For the first Accumulated damage over time This is the equivalent creep strain increment. This represents the critical fracture strain under the current stress state. It is the critical ductility coefficient. It is a stress triaxiality.

2. The method for predicting the risk zone of heat treatment reheat cracking according to claim 1, characterized in that, In step S1, the component to be analyzed is a high-temperature pressure-bearing component with welded joints, and the component to be analyzed has residual welding stress.

3. The method for predicting the risk zone of reheat cracking after heat treatment according to claim 1, characterized in that, In step S3, the load constraint is applied in the form of a displacement constraint and used to simulate the stress relaxation behavior of the component to be analyzed under structural constraints during heat treatment.

4. The method for predicting the risk zone of reheat cracking after heat treatment according to claim 1, characterized in that, The heat treatment damage activation condition set in step S3 is as follows: Current temperature > preset activation temperature threshold.

5. The method for predicting the risk zone of reheat cracking after heat treatment according to claim 1, characterized in that, The method for determining the dangerous zone of heat treatment reheat cracking in step S4 is as follows: Set a cumulative damage threshold, and use the area in the damage field distribution cloud map obtained in step S3 where the cumulative damage reaches the cumulative damage threshold as the dangerous zone for heat treatment reheat cracking.

6. The method for predicting the risk zone of heat treatment reheat cracking according to claim 5, characterized in that, It also includes determining the first cracking zone based on the damage field distribution cloud map obtained in step S3, specifically: the area where the maximum cumulative damage value is located in the damage field distribution cloud map obtained in step S3 is taken as the first cracking zone.

7. The method for predicting the risk zone of heat treatment reheat cracking according to claim 5, characterized in that, It also includes determining the potential crack initiation path based on the damage field distribution cloud map obtained in step S3, specifically: performing connectivity identification on the damage field distribution cloud map obtained in step S3 to determine the potential crack initiation path.

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