A method for evaluating the creep life of a weld structure heat affected zone crack

Abstract

The present application belongs to the technical field of service structure safety evaluation, and in particular to a creep life evaluation method for cracks in the heat affected zone of a welded structure, comprising the following steps: step one, determining the material parameters of the welded structure; step two, constructing a finite element model of the heat affected zone of the welded joint containing different lengths of initial cracks; step three, determining the boundary conditions of the welded structure; step four, simulating the creep damage, determining the maximum creep damage position of each crack front and the creep life; and step five, constructing a correlation between crack size and corresponding creep life. The creep life evaluation method for cracks in the heat affected zone of a welded structure simulates the creep damage evolution process of the welded structure under different initial crack lengths, constructs a quantitative functional relationship between crack length and creep life, and uses this relationship to predict the creep life of the welded structure by only obtaining the crack length in the structure, significantly improving the engineering applicability and efficiency of life evaluation.

Description

Technical Field

[0001] This invention relates to the field of service structure safety evaluation technology, and in particular to a method for assessing the creep life of cracks in the heat-affected zone of welded structures. Background Technology

[0002] The core load-bearing components of major equipment in aerospace, energy and power, petrochemical, and marine engineering are mostly welded structures, and their service performance directly determines the safety, reliability, and economy of the entire equipment. However, welded joints exhibit significant regional inhomogeneities in microstructure and mechanical properties, and the welding process inevitably introduces metallurgical defects such as porosity, slag inclusions, and lack of fusion. These defects are considered initial cracks, making the welded zone (especially the heat-affected zone) a weak link in the assessment of high-temperature creep failure. Currently widely used creep life prediction methods (such as the Larson-Miller parameter method, the Orr-Sherby-Dorn parameter method, and the isotherm extrapolation method) obtain extrapolation curves of materials under different stresses by fitting parameters in the model through a large number of long-term creep experiments. For welded structures, creep damage simulation is usually performed using the method of continuous damage mechanics. In the simulation, each region of the welded joint is generally regarded as a homogeneous continuum without initial cracks, without considering the unavoidable welding defects in the actual welded structure, which are usually regarded as initial cracks. Since the presence of such initial cracks significantly affects the initiation and evolution of creep damage, ignoring their existence will lead to a large deviation between the predicted lifespan and the actual engineering results. Therefore, fully considering the impact of initial cracks in the weld heat-affected zone on the creep life of the structure and achieving accurate prediction of the creep life in this region is of great engineering significance for ensuring the reliability assessment of welded structures under high-temperature service conditions.

[0003] While existing technologies exist for predicting the creep life of welded structures, their accuracy and applicability remain insufficient. For example, the patent technology in CN116050228B introduces the influence of microstructure degradation on P92 steel to predict creep life, but its application is limited because the phenomenon of rapid decrease in creep strength due to microhardness is not universally applicable. The patent technology in CN114282411B, combining restraint parameters, establishes a relationship between weld joint mismatch parameters and the creep crack propagation rate of the weld center crack. However, this method requires obtaining the remaining ligament length at the crack front for creep life prediction, a parameter difficult to determine for actual welded structures, limiting its application to standard specimens. Summary of the Invention

[0004] Existing methods are either limited to specific materials or lack the necessary parameters for structural assessment, resulting in a lack of a universally applicable method for predicting the creep life of the heat-affected zone (HAZ) that directly incorporates the influence of initial crack length. This invention proposes a method for assessing the creep life of cracks in the HAZ of welded structures. It fully considers the regional inhomogeneity of welded structures, embeds continuous damage mechanics into finite element simulation, and establishes a quantitative functional relationship between the initial crack length and creep life. The input parameter for this relationship is only the crack size, eliminating the need for additional complex tests or difficult-to-obtain field parameters. This method can quickly and effectively assess the remaining life of in-service high-temperature welded structures, providing a directly applicable quantitative indicator for on-site maintenance and replacement decisions.

[0005] The present invention proposes a method for assessing the creep life of cracks in the heat-affected zone of welded structures, comprising step one: determining the material parameters of the welded structure;

[0006] Step 2: Construct finite element models of welded structures with different crack lengths within the heat-affected zone;

[0007] Step 3: Determine the boundary conditions of the welded structure;

[0008] Step 4: Perform creep damage simulation to determine the location of maximum creep damage and creep life for each crack;

[0009] Step 5: Construct the correlation between crack length and corresponding creep life.

[0010] Preferably, the material parameters include elastic modulus E, Poisson's ratio μ, and yield strength σ. 0.2 Power-law creep parameter A, creep exponent n, and uniaxial creep ductility ε f .

[0011] Preferably, the finite element model of the welded structure constructed in step two includes the weld zone, the heat-affected zone, and the base material zone.

[0012] Preferably, the width of the heat-affected zone is determined by the width in the actual structure, and the crack length is determined by the width of the heat-affected zone, with at least five crack length values ​​taken to ensure the accuracy of the constructed relationship and life prediction.

[0013] Preferably, in step three, the boundary conditions of the welded structure are applied based on the actual stress state of the structure under service conditions, and creep damage finite element simulation is performed by combining the secondary development of the finite element subroutine.

[0014] Preferably, when the welded structure is subjected to internal pressure in a pressure vessel, the stress on the cross section (i.e., the axial stress sL) and the circumferential stress st on the longitudinal section are determined by the internal pressure:

[0015] ;

[0016]

[0017] Where P is the internal pressure, D is the outer diameter, and t is the thickness. To accurately calculate the creep life of the welded structure, the axial and circumferential stresses calculated under the internal pressure are applied to the finite element model.

[0018] Preferably, in step four, creep damage variables are embedded into the material properties through a finite element software subroutine to simulate creep damage.

[0019] Preferably, the creep damage simulation adopts a strain-based creep damage model. The position where the cumulative creep damage first reaches 1 is regarded as the position with the largest cumulative creep damage at the crack front. The creep time at this time is the creep life. In actual creep damage simulation, in order to avoid numerical singularity in the calculation process, the creep time when the cumulative creep damage is 0.99 is taken as the creep life.

[0020] Preferably, the assessment of the creep damage amount is based on the relationship between the local creep strain accumulation and creep ductility. A subroutine is used to correlate the creep damage amount with the elastic modulus of the material to simulate unit failure and inability to bear the load, causing the crack to propagate forward.

[0021] Preferably, based on the simulation results of different crack lengths carried out in step five, the relationship between crack length and creep life is constructed as shown in the following formula:

[0022]

[0023] Where t0, A1, A2, t1, and t2 are the fitting parameters, and t = t m / t r a=a m / a r , t m a m These are the current creep life and the corresponding crack length, t. r a r These are the reference creep life and the corresponding reference crack length selected after normalization processing, respectively. The reference crack length is selected by taking the shortest crack length and the corresponding creep life as the reference.

[0024] The beneficial effects of this invention are as follows:

[0025] The creep life assessment method for welded structures considering heat-affected zone cracks proposed in this invention can achieve two engineering applications by establishing a quantitative relationship between crack length and creep life: first, to quickly predict the remaining creep life based on the crack length obtained from the detection of in-service structures; second, to reverse-calculate the maximum allowable crack size based on the designed or specified service life, thereby determining whether the current crack has entered the failure propagation stage. Attached Figure Description

[0026] Figure 1 This is a flowchart of a method for assessing the creep life of cracks in the heat-affected zone of a welded structure, as proposed in this invention.

[0027] Figure 2 This is a finite element model of a welded structure containing heat-affected zone cracks, which is part of the creep life assessment method for heat-affected zone cracks in welded structures proposed in this invention.

[0028] Figure 3 This is a creep damage cloud map of the crack front at different crack lengths for a method to assess the creep life of cracks in the heat-affected zone of a welded structure proposed in this invention.

[0029] Figure 4 This is a diagram showing the evolution of creep damage over time under different crack lengths in a method for assessing the creep life of cracks in the heat-affected zone of a welded structure, as proposed in this invention.

[0030] Figure 5 This is a curve showing the relationship between crack length and creep life obtained by fitting the method for evaluating the creep life of cracks in the heat-affected zone of a welded structure proposed in this invention. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0032] Reference Figure 1-5 A method for assessing the creep life of cracks in the heat-affected zone of welded structures, such as... Figure 1 As shown, the process includes step one: determining the material parameters of the welded structure. The welded structure is divided into a weld zone, a heat-affected zone, and a base metal zone. The material parameters for each zone are obtained through uniaxial tensile testing and uniaxial tensile creep testing. These material parameters include the elastic modulus E, Poisson's ratio μ, and yield strength σ. 0.2 Power-law creep parameter A, creep exponent n, and uniaxial creep ductility ε f .

[0033] In this embodiment, creep damage simulation was conducted using the material properties of P92 steel at 650 °C. The material properties of each region of the welded structure are shown in the table below:

[0034]

[0035] The creep constitutive equations for materials in each region are expressed using Norton constitutive methods:

[0036] ;

[0037] Among them, A i n i These are the creep constant and creep index for the corresponding regions, respectively.

[0038] Step 2: Construct finite element models with different crack lengths, including the weld zone, heat-affected zone, and base metal zone, with the crack located at the center of the heat-affected zone.

[0039] In the finite element model of the welded structure with heat-affected zone (HAZ) cracks constructed in this example, the cracks in the structure are generally represented as semi-ellipses or semi-circles. In this example, to improve computational efficiency, the cracks are treated as semi-circles, and the crack location is located at the center of the HAZ on the surface of the welded structure. Finite element models for six crack lengths were constructed. The width of the HAZ is determined by the width specified in the actual structure. To ensure the accuracy of the prediction, the range of crack lengths should cover the width of the HAZ as much as possible.

[0040] In this embodiment, the welded structure model is constructed as a 9 mm × 9 mm × 9 mm cube, wherein the widths of the weld, heat-affected zone, and base material are all equal to 3 mm. The crack lengths within the heat-affected zone are taken as 0.3 mm, 0.8 mm, 1.2 mm, 1.6 mm, 2.0 mm, and 2.4 mm. The constructed finite element model and the mesh generation of the crack front are as follows. Figure 2 As shown.

[0041] Step 3: Determine the boundary conditions of the welded structure; the application of boundary conditions in the finite element model should be based on the equivalent environment of the welded structure under operating conditions. For the welded structure in a pressure vessel subjected to internal pressure, the stress on the cross-section, i.e., the axial stress s, is... L and the circumferential stress s on the longitudinal section t It is determined by the internal pressure it receives.

[0042] ;

[0043] ;

[0044] Where P is the internal pressure, D is the diameter, and t is the thickness. To accurately calculate the creep life of the welded structure, the axial and circumferential stresses calculated under the working internal pressure are applied to the finite element model.

[0045] In this embodiment, to simplify calculations, an equiaxial biaxial stress state is adopted, and a uniformly distributed load of 30 MPa is applied to both directions of the model. In practical applications, the load should be adjusted according to the stress ratio. Symmetrical boundary conditions are applied to the ligament at the crack tip and the bottom surface adjacent to the crack, such as... Figure 2 As shown.

[0046] Step 4: Perform creep damage simulation. A strain-based creep damage model can be used. The creep damage amount is evaluated based on the relationship between the local creep strain accumulation and creep ductility. A subroutine is used to correlate the creep damage amount with the material's elastic modulus to simulate element failure and inability to bear load, causing crack propagation. This determines the location with the maximum cumulative creep damage at the crack front. In creep damage simulation, the location with the higher creep damage at the crack front for the same creep time is the location with the maximum cumulative creep damage. In actual creep damage simulation, to avoid numerical singularities in the calculation process, the creep life is defined as the creep time when the cumulative creep damage amount is 0.99.

[0047] In this embodiment, the creep damage amount ω is correlated with the elastic modulus of the heat-affected zone material using the finite element software ABAQUS, the creep subroutine CREEP, and the field variable subroutine USDFLD to simulate material degradation caused by the accumulation of creep damage. The accumulation of creep damage is calculated using the following formula:

[0048] ;

[0049] in, The creep strain rate, It exhibits multiaxial creep ductility.

[0050] The relationship between multiaxial creep ductility and uniaxial creep ductility is described using the Cocks-Asheby model: ;

[0051] in, It is hydrostatic stress. It is the equivalent Mises stress.

[0052] The finite element simulation results of creep damage are as follows: Figure 2 As shown, it can be observed from the creep damage distribution at the crack front that the damage is greatest near the crack surface point, and the creep damage gradually decreases from the crack surface point to the deepest point. The simulation results are similar for different crack lengths. Therefore, the time when creep failure is reached at this location is taken as the creep time of the structure.

[0053] Step 5: Obtain the creep life of cracks of different lengths;

[0054] In actual creep damage simulation, to avoid numerical singularities in the calculation process, the creep time when the cumulative creep damage is 0.99 is taken as the creep failure life. Based on the simulation results of different crack lengths carried out in step five, the relationship between crack length and creep life is as follows:

[0055] In this example, the creep life under different crack lengths was simulated as follows: Figure 3 As shown, the damage accumulation rate gradually increases with the increase of crack length. Under the same crack length, the creep damage accumulation rate gradually decreases. In the early stage of accumulation, the damage amount rapidly increases to a high value, and then the accumulation slows down with the increase of creep time.

[0056] Furthermore, by fitting the material parameters, a correlation between crack length and creep life is constructed. Based on the simulation results, the material constants in the following formula are obtained.

[0057] ;

[0058] Where t0, A1, A2, t1, and t2 are the parameters obtained from the fitted curve; t = t m / t r a=a m / a r , t m a m These are the current creep life and the corresponding crack length, t. r a r These are the baseline creep life and the corresponding baseline crack length selected after normalization. Generally, the shortest crack length and its corresponding creep life are chosen as the baseline. This formula effectively describes the phenomenon that "the impact is significant in the small crack stage and gradually diminishes in the large crack stage."

[0059] In this embodiment, y0=2.23, A1=-1.47, t1=0.69, A2=0.79, t2=6.49, t m =2.7×10 4 h, a m =0.3 mm.

[0060] The above embodiments employ a creep damage simulation method. By constructing a finite element model of a P92 steel welded structure with heat-affected zone cracks, the crack length and creep failure time are correlated, enabling the prediction of creep life under different crack lengths in the welded structure. This is of great significance for accurately assessing the life of actual welded structural components under high-temperature creep conditions.

[0061] This invention establishes a quantitative relationship between crack length and creep life, enabling two engineering applications: first, to quickly predict the remaining creep life of an in-service structure based on the crack length detected in the test; second, to reverse-calculate the maximum allowable crack size based on the designed or specified service life, thereby determining whether the current crack has entered the failure propagation stage.

[0062] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for assessing the creep life of cracks in the heat-affected zone of a welded structure, characterized in that: This includes step one: determining the material parameters for the welded structure; Step 2: Construct finite element models of welded structures with different crack lengths within the heat-affected zone; Step 3: Determine the boundary conditions of the welded structure; Step 4: Perform creep damage simulation to determine the location of maximum creep damage and creep life for each crack; Step 5: Construct the correlation between crack length and corresponding creep life.

2. The method for assessing the creep life of heat-affected zone cracks in welded structures according to claim 1, characterized in that: The material parameters include elastic modulus E, Poisson's ratio μ, and yield strength σ. 0.2 Power-law creep parameter A, creep exponent n, and uniaxial creep ductility ε f .

3. The method for assessing the creep life of heat-affected zone cracks in welded structures according to claim 1, characterized in that: The finite element model of the welded structure constructed in step two includes the weld zone, the heat-affected zone, and the base material zone.

4. The method for assessing the creep life of heat-affected zone cracks in welded structures according to claim 3, characterized in that: The width of the heat-affected zone is determined by the width defined in the actual structure, and the crack length is determined by the width of the heat-affected zone, with at least five crack length values ​​taken to ensure the accuracy of the constructed relationship and life prediction.

5. The method for assessing the creep life of heat-affected zone cracks in welded structures according to claim 1, characterized in that: In step three, the boundary conditions of the welded structure are applied based on the actual stress state of the structure under service conditions. Combined with the secondary development of the finite element subroutine, creep damage finite element simulation is performed.

6. The method for assessing the creep life of heat-affected zone cracks in welded structures according to claim 5, characterized in that: When the welded structure is subjected to internal pressure in a pressure vessel, the stress on its cross-section, i.e., the axial stress s, is... L and the circumferential stress s on the longitudinal section t Determined by the internal pressure it experiences: ； ； Where P is the internal pressure, D is the outer diameter, and t is the thickness. To accurately calculate the creep life of the welded structure, the axial and circumferential stresses calculated under the internal pressure are applied to the finite element model.

7. The method for assessing the creep life of heat-affected zone cracks in welded structures according to claim 1, characterized in that: In step four, creep damage variables are embedded into the material properties through a finite element software subroutine to simulate creep damage.

8. The method for assessing the creep life of heat-affected zone cracks in welded structures according to claim 7, characterized in that: The creep damage simulation adopts a strain-based creep damage model. The position where the cumulative creep damage first reaches 1 is regarded as the position with the largest cumulative creep damage at the crack front. The creep time at this point is the creep life. In actual creep damage simulation, in order to avoid numerical singularity in the calculation process, the creep time when the cumulative creep damage is 0.99 is taken as the creep life.

9. The method for assessing the creep life of heat-affected zone cracks in welded structures according to claim 8, characterized in that: The assessment of creep damage is based on the relationship between local creep strain accumulation and creep ductility. A subroutine is used to correlate creep damage with the elastic modulus of the material to simulate unit failure and inability to bear load, causing cracks to propagate forward.

10. The method for assessing the creep life of heat-affected zone cracks in welded structures according to claim 1, characterized in that: Based on the simulation results of different crack lengths carried out in step five, the relationship between crack length and creep life is constructed as shown in the following formula: ； Where t0, A1, A2, t1, and t2 are the fitting parameters, and t = t m / t r a=a m / a r , t m a m These are the current creep life and the corresponding crack length, t. r a r These are the baseline creep life and the corresponding baseline crack length selected for normalization.

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