Nondestructive evaluation method for residual life of P92 steel steam pipeline based on in-situ micron press-in

Abstract

A P92 steel steam pipeline residual life nondestructive evaluation method based on in-situ micron indentation comprises the steps that a portable micron indenter is used for in-situ testing to obtain the indentation elastic modulus and the indentation creep stress index of a P92 steel steam pipeline, and corresponding damage variables are constructed by combining the elastic modulus and the indentation creep stress index of a material when the material leaves a factory; according to the method, damage variables are introduced to correct parameters in a K-R equation, so that multi-damage variable parameter coupling identification of the endurance strength of the material is realized, and the residual life of the material is further estimated based on the endurance strength obtained through identification. According to the method, damage variables are introduced to correct Kachanov-Robatnov (hereinafter referred to as K-R) equation parameters, and nondestructive identification of the endurance strength is realized in combination with a micron indentation technology, so that an accurate and efficient residual life evaluation model suitable for the field is established; the limitation of a traditional method can be effectively overcome, and a nondestructive evaluation technology for the residual life of the in-service pipeline is provided.

Description

Technical Field

[0001] The present invention belongs to the technical field of material performance evaluation and nondestructive testing, and specifically relates to a method for evaluating the remaining life of a P92 steel steam pipeline based on micron indentation technology and a damage variable correction model. The method is suitable for life prediction and health status monitoring of P92 steel steam pipelines in service in thermal power units. Background Art

[0002] Ultra-supercritical (UFC) units are widely used in the modern thermal power industry, favored for their high efficiency and low emissions. As a key material for the steam piping of UFC units, the long-term service life of P92 steel directly impacts the unit's safety and reliability. However, due to the long-term exposure of steam piping to high temperatures and high pressures, the material is subject to creep, oxidation, and accumulated damage, leading to a gradual deterioration of its mechanical properties and ultimately failure. Therefore, accurately assessing the remaining life of steam piping is crucial to ensuring safe unit operation.

[0003] Existing methods for assessing the remaining life of P92 steel fall into three main categories: creep life model prediction methods, microstructure assessment methods, and flaw detection methods. Creep life model methods are typically based on experimental data or damage mechanics models, predicting the long-term life of the material by extrapolating or establishing creep life relationships. Although these methods offer high accuracy when sufficient data is available, they often rely on long-term endurance testing using pipe cutting and sampling, which results in long test cycles and high costs. Microstructure assessment methods assess the degree of material degradation by analyzing microstructural changes (such as precipitates and grain boundary damage), but the experimental process is complex and difficult to perform real-time online testing. Flaw detection methods, primarily utilizing ultrasonic testing, magnetic memory testing, and acoustic emission techniques, assess life by detecting defects such as cracks, holes, and damage in serving structures. While offering the advantages of non-destructive testing, these methods suffer from limited resolution and significant environmental impact, resulting in limitations in detection accuracy and quantitative analysis capabilities.

[0004] In recent years, instrumented indentation technology, a technique for testing micro-area surface mechanical properties, has been widely used in material mechanical property testing. Unlike traditional hardness testing, instrumented indentation technology can infer the mechanical properties of a material by recording the relationship between indentation load and depth. Portable micron indenters based on micron indentation offer significant advantages in environmental adaptability and ease of operation, making them particularly suitable for on-site nondestructive testing. However, existing technologies have not yet effectively combined micron indentation technology with residual life assessment models, making it difficult to accurately assess the remaining life of P92 steel steam pipelines. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, such as long processing times, poor efficiency, limited applicability, and low accuracy, the present invention provides an in-service, non-destructive, rapid, convenient, and effective method for non-destructive assessment of the remaining life of P92 steel steam pipelines based on in-situ micron indentation. By introducing damage variables to modify the parameters of the Kachanov-Robatnov (KR) equation and combining micron indentation technology to achieve non-destructive identification of endurance strength, an accurate, efficient, and field-applicable remaining life assessment model is established. This method effectively overcomes the limitations of traditional methods and provides a non-destructive technology for assessing the remaining life of in-service pipelines.

[0006] To achieve the above object, the present invention proposes the following technical solutions:

[0007] A nondestructive assessment method for the remaining life of a P92 steel steam pipeline based on in-situ micron indentation is proposed. A portable micron indentation instrument is used for in-situ testing to obtain the indentation elastic modulus and indentation creep stress index of the P92 steel steam pipeline. The corresponding damage variables are then constructed by combining the elastic modulus and indentation creep stress index of the material at factory ex-factory (initial material). The parameters in the KR equation are modified by introducing the damage variables, thereby achieving coupled identification of multiple damage variable parameters for the material's endurance strength. The remaining life of the material is then estimated based on the endurance strength obtained by identification.

[0008] Furthermore, the method comprises the following steps:

[0009] In the first step, the damage variable constructed based on the micron indentation test parameters is shown in formula (1):

[0010]

[0011] Where, are the damage variables constructed based on elastic modulus and indentation creep stress index, E0, n IT0 is the elastic modulus and indentation creep stress index of the P92 steel steam pipe material in the initial state when leaving the factory, E IT 、n IT The indentation elastic modulus and indentation creep stress index of the component to be evaluated obtained through micron indentation testing;

[0012] In the second step, the parameters of the KR creep constitutive equation are used to describe the parameters of the power-law equation for endurance strength, and damage variables are introduced to modify the KR creep equation parameters to achieve coupled identification of multiple damage variable parameters for endurance strength. The KR creep constitutive equation is a classic constitutive model that describes the creep damage evolution of materials at high temperatures, which is described as Equation (2):

[0013]

[0014] Where, is the creep rate, σ is the stress load borne by the material, is the damage evolution rate, D is the damage variable, A is the creep constant related to material and temperature, n is the creep stress exponent, and B is the material parameter that controls the damage evolution rate. is the damage evolution index, where the damage variable D∈[0,1) quantitatively characterizes the evolution of the material's damage degree. The initial values t=0 and D=0 are set to represent the material as initially undamaged upon leaving the factory. Based on the initial conditions, the damage evolution equation is integrated with separated variables to obtain the relationship between the material's service life t and the damage variable D:

[0015]

[0016] According to the definition of damage variable D, when the material reaches the critical damage state, D→1, and the service time at this time is the fracture time t of the material. r , so the relationship between the stress load and fracture time of the material is obtained as formula (4):

[0017]

[0018] According to the isothermal extrapolation method recommended in the standard DL / T 940-2022, the relationship between different stresses and creep rupture time of the material at the same test temperature is described by the power law equation shown in formula (5):

[0019] σ=kt r m (5)

[0020] Where σ is the applied stress load, t r is the fracture time, k is the endurance strength coefficient, and m is the time sensitivity index. Formula (5) is used to fit the creep endurance experimental data and extrapolate to obtain the endurance strength (This value indicates the test temperature T at a specified time t r The lasting strength, such as Indicates that the duration of 10 4 Hours of endurance strength), the power law equation parameters of the endurance strength described by the KR creep constitutive equation parameters are obtained by combining equations (5) and (4):

[0021]

[0022] Parameter B, It is a variable that depends on the aging state of the material and evolves with the aging of the material. By introducing the damage variable D to describe the parameters of the KR creep equation, the hypothesis is established:

[0023]

[0024] B=B'(D) (8)

[0025] Based on equations (7) and (8), the KR equation is modified so that the material parameters in the equation are adjusted with the damage evolution, which is more in line with the actual physical process. Based on the creep endurance test results of P92 steel materials with different aging degrees and the micron indentation test results, the following is fitted and analyzed: The specific mathematical form of B'(D):

[0026]

[0027] Where, B0 is the parameter of the initial material when leaving the factory. The coefficient is obtained by fitting, and the material damage variable is substituted into formula (9) and formula (10), and then combined with formula (6) to realize the multi-damage variable parameter coupling identification of the material endurance strength, that is, the material service temperature T is obtained by analyzing the mechanical properties parameters of micron indentation. 4 and 10 5 Extrapolated endurance strength corresponding to hours

[0028] In the third step, the remaining life of the P92 steel steam pipeline is further evaluated according to the creep life calculation formula shown in formula (11):

[0029]

[0030] Where, t re is the remaining life, σ max It is the maximum value between the internal pressure reduced stress and the hoop thermal stress of the pipeline. They are the extrapolated endurance strength of 10,000 and 100,000 hours at temperature T, respectively. η is the safety factor, and in the present invention, η=1.5.

[0031] Furthermore, in the third step, according to the requirements of standard DL / T 940-2022:

[0032] a) Hoop thermal stress σ h Estimated by the following formula:

[0033]

[0034] Where σ h is the hoop thermal stress, E h is the elastic modulus of the material at the service temperature, α is the linear expansion coefficient of the material, ν is the Poisson's ratio of the material, ΔT is the temperature difference between the outer and inner walls of the pipe, and f is the structural coefficient. The structural coefficient is closely related to the geometric parameters of the pipe and its expression is as follows:

[0035]

[0036] Where β is the ratio of the outer diameter to the inner diameter of the pipe determined according to the nominal thickness;

[0037] b) Internal pressure reduced stress σ eq Estimated by the following formula:

[0038]

[0039] Where σ eq is the internal pressure converted stress, p is the pipeline operating pressure, D o is the outer diameter of the pipe, S is the wall thickness of the pipe; Y is the temperature correction coefficient for the wall thickness calculation formula, and a is the additional wall thickness considering corrosion, wear and mechanical strength;

[0040] The nondestructive assessment model for remaining useful life based on micron indentation testing proposed in the present invention is as follows:

[0041]

[0042] This model converts the indentation elastic modulus E obtained from the micron indentation experiment into IT and indentation creep stress index n IT As input, construct the damage variable By establishing a modified KR parameter function model and B'(D) to find and B, and further obtain the endurance strength parameters k and m, and extrapolate to obtain Finally, combined with the maximum service stress σ max Calculate the remaining life t re This will then complete the prediction of the remaining life of the material.

[0043] The technical concept of this invention is as follows: Through experimental research and mathematical analysis, it was found that introducing damage variables into the parameters of the KR creep constitutive equation can better describe the damage evolution process during the material's service life. This theory is then combined with the power-law equation describing the endurance strength to achieve endurance strength identification based on multiple damage variable parameters. Furthermore, by combining in-situ micron indentation technology with a method for calculating the remaining life based on endurance strength, a nondestructive assessment method for the remaining life of P92 steel steam pipelines based on in-situ micron indentation can be established, providing a nondestructive, more accurate, and efficient testing method for remaining life assessment and health monitoring of steam pipelines.

[0044] The beneficial effects of the present invention are demonstrated by performing efficient and convenient in-situ micron-indentation testing on the pipeline under test, enabling the acquisition of its indentation mechanical performance parameters without destroying the pipeline. Substituting the acquired test data into the theoretical method proposed in this invention for calculation, the remaining life of the pipeline under test can be determined. Compared to existing methods for assessing the remaining life of P92 steel steam pipelines, this method not only improves the accuracy and efficiency of non-destructive testing life assessments, but also addresses the high cost, long cycle time, and destructive testing issues inherent in traditional methods, demonstrating significant engineering application value and market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a flow chart of a nondestructive assessment method for the remaining life of a P92 steel steam pipeline based on in-situ micron indentation according to the present invention;

[0046] Figure 2 It is a schematic diagram of the evaluation path of the nondestructive evaluation model for the remaining life of a P92 steel steam pipeline established in the present invention. DETAILED DESCRIPTION

[0047] The present invention will be further described below with reference to the accompanying drawings.

[0048] Reference Figure 1 and Figure 2 A nondestructive assessment method for the remaining life of a P92 steel steam pipeline based on in-situ micron indentation is proposed. A portable micron indentation instrument is used for in-situ testing to obtain the indentation elastic modulus and indentation creep stress index of the P92 steel steam pipeline. The corresponding damage variables are constructed by combining the elastic modulus and indentation creep stress index of the material at the time of leaving the factory (initial material). By introducing the damage variables, the parameters in the KR equation are corrected, thereby realizing the coupled identification of multiple damage variable parameters for the material's endurance strength. The remaining life of the material is further estimated based on the endurance strength obtained by identification.

[0049] This example tests several groups of P92 steel pipeline materials in different aging states to verify the non-destructive assessment method for the remaining service life of P92 steel steam pipelines based on in-situ micron indentation.

[0050] The steps for conducting non-destructive assessment of the remaining service life of P92 steel steam pipelines are as follows:

[0051] 1) Determine the specifications of the P92 steel pipeline to be evaluated (including the outer diameter D o ; pipe wall thickness S; ratio of pipe outer diameter to inner diameter β; additional wall thickness a considering corrosion, wear and mechanical strength, etc.), service parameters (including operating pressure p; service temperature T; temperature difference between inner and outer walls ΔT during service, etc.), initial material mechanical properties parameters at factory (including initial elastic modulus E0; initial indentation creep stress index n IT0 ; Elastic modulus E at service temperature hNecessary data for life assessment, such as linear expansion coefficient α and Poisson's ratio ν;

[0052] 2) Based on the layout and structure of the pipeline, select multiple representative pipe sections and formulate a reasonable in-situ micron indentation test plan;

[0053] 3) According to the test location and the test plan, a portable micron indentation test was carried out to obtain the indentation mechanical performance parameters of the in-service P92 steel pipeline (including the indentation elastic modulus E IT ; Indentation creep stress index n IT wait);

[0054] 4) Based on the obtained indentation mechanical properties parameters and the mechanical properties parameters of the initial material at the factory, the damage variables based on the elastic modulus are constructed using formula (1). and damage variables based on the indentation creep stress index

[0055] 5) Based on the endurance strength multi-damage variable parameter coupling identification method proposed in the present invention, the damage variables obtained in step 4) are and Substituting into equations (9) and (10) we can get the KR equation parameters: and B, and then Substitute k and m into formula (6), and further combine formula (5) to calculate the extrapolated endurance strength of the P92 steel pipeline to be evaluated at the service temperature T

[0056] 6) According to the remaining life assessment model, the hoop thermal stress σ is calculated using formula (12) h , Equation (13) and Equation (14) calculate the internal pressure reduced stress σ eq , take the maximum value between the hoop thermal stress and the internal pressure converted stress as σ max , further using the extrapolated endurance strength obtained in step 5) Combined with formula (11), the remaining life of the P92 steel pipeline is estimated.

[0057] The nondestructive remaining life assessment method for P92 steel steam pipelines based on in-situ micron indentation proposed in the present invention was used to assess the life of three P92 steel pipeline materials in different aging states obtained through accelerated aging experiments. In addition, the three materials were subjected to endurance strength tests combined with the isothermal extrapolation method to obtain the remaining life assessment results. The results were compared with the prediction results of the method of the present invention. The comparison results are shown in Table 1:

[0058]

[0059] Table 1

[0060] The maximum relative error of the comparison result is less than 15.5%, which proves that the life assessment method proposed in the present invention can efficiently and accurately predict the remaining life of P92 steel steam pipelines.

[0061] The embodiments of this specification are merely examples of implementations of the invention and are provided for illustrative purposes only. The scope of protection of the present invention should not be considered limited to the specific embodiments described in these embodiments. The scope of protection of the present invention also extends to equivalent technical means that can be conceived by a person of ordinary skill in the art based on the invention.

Claims

1. A nondestructive assessment method for the remaining life of a P92 steel steam pipeline based on in-situ micron indentation, characterized in that: A portable micron indenter was used for in-situ testing to obtain the indentation elastic modulus and indentation creep stress index of the P92 steel steam pipeline. The corresponding damage variables were constructed by combining the elastic modulus and indentation creep stress index of the material at the factory. The parameters in the KR equation were modified by introducing the damage variables, thereby realizing the coupled identification of multiple damage variable parameters for the material's endurance strength. The remaining life of the material was further estimated based on the endurance strength obtained by identification.

2. The nondestructive assessment method for the remaining life of a P92 steel steam pipeline based on in-situ micron indentation according to claim 1, characterized in that: The method comprises the following steps: In the first step, the damage variable constructed based on the micron indentation test parameters is shown in formula (1): Where, are the damage variables constructed based on elastic modulus and indentation creep stress index, E0, n IT0 is the elastic modulus and indentation creep stress index of the P92 steel steam pipe material in the initial state when leaving the factory, E IT 、n IT The indentation elastic modulus and indentation creep stress index of the component to be evaluated obtained through micron indentation testing; In the second step, the parameters of the KR creep constitutive equation are used to describe the parameters of the power-law equation for endurance strength, and damage variables are introduced to modify the KR creep equation parameters to achieve coupled identification of multiple damage variable parameters for endurance strength. The KR creep constitutive equation is a classic constitutive model that describes the creep damage evolution of materials at high temperatures, which is described as Equation (2): Where, is the creep rate, σ is the stress load borne by the material, is the damage evolution rate, D is the damage variable, A is the creep constant related to material and temperature, n is the creep stress exponent, and B is the material parameter that controls the damage evolution rate. is the damage evolution index, where the damage variable D∈[0,1) quantitatively characterizes the evolution of the material damage degree. The initial values are set to t=0, D=0, indicating that the material is initially undamaged at the factory. Based on the initial conditions, the damage evolution equation is separated and integrated to obtain the relationship between the material service time t and the damage variable D: According to the definition of damage variable D, when the material reaches the critical damage state, D→1, and the service time at this time is the fracture time t of the material. r , so the relationship between the stress load and fracture time of the material is obtained as formula (4): According to the isothermal extrapolation method recommended in the standard DL / T 940-2022, the relationship between different stresses and creep rupture time of the material at the same test temperature is described by the power law equation shown in formula (5): σ=kt r m (5) Where σ is the applied stress load, t r is the fracture time, k is the endurance strength coefficient, and m is the time sensitivity index. Formula (5) is used to fit the creep endurance experimental data and extrapolate to obtain the endurance strength Combining equations (5) and (4) yields the parameters of the power-law equation for the endurance strength described by the KR creep constitutive equation: Parameter B, It is a variable that depends on the aging state of the material and evolves with the aging of the material. By introducing the damage variable D to describe the parameters of the KR creep equation, the hypothesis is established: B=B'(D) (8) Based on equations (7) and (8), the KR equation is modified so that the material parameters in the equation are adjusted with the damage evolution, which is more in line with the actual physical process. Based on the creep endurance test results of P92 steel materials with different aging degrees and the micron indentation test results, the following is fitted and analyzed: The specific mathematical form of B'(D): Where, B0 is the parameter of the initial material when leaving the factory. The coefficient is obtained by fitting, and the material damage variable is substituted into formula (9) and formula (10), and then combined with formula (6) to realize the multi-damage variable parameter coupling identification of the material endurance strength, that is, the material service temperature T is obtained by analyzing the mechanical properties parameters of micron indentation. 4 and 10 5 Extrapolated endurance strength corresponding to hours In the third step, the remaining life of the P92 steel steam pipeline is further evaluated according to the creep life calculation formula shown in formula (11): Where, t re is the remaining life, σ max It is the maximum value between the internal pressure reduced stress and the hoop thermal stress of the pipeline. are the extrapolated endurance strengths of 10,000 and 100,000 hours at temperature T, respectively, and η is the safety factor.

3. The nondestructive assessment method for the remaining life of a P92 steel steam pipeline based on in-situ micron indentation according to claim 2, characterized in that: In the third step, according to the requirements of standard DL / T 940-2022: a) Hoop thermal stress σ h Estimated by the following formula: Where σ h is the hoop thermal stress, E h is the elastic modulus of the material at the service temperature, α is the linear expansion coefficient of the material, ν is the Poisson's ratio of the material, ΔT is the temperature difference between the outer wall and the inner wall of the pipe, and f is the structural coefficient. The structural coefficient is closely related to the geometric parameters of the pipe. Its expression is as follows: Where β is the ratio of the outer diameter to the inner diameter of the pipe determined according to the nominal thickness; b) Internal pressure reduced stress σ eq Estimated by the following formula: Where σ eq is the internal pressure converted stress, p is the pipeline operating pressure, D o is the outer diameter of the pipe, S is the wall thickness of the pipe; Y is the temperature correction coefficient of the wall thickness calculation formula, and a is the additional wall thickness considering corrosion, wear and mechanical strength.

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