A high-temperature creep life prediction method based on creep deformation mechanism

Abstract

This invention provides a high-temperature creep life prediction method based on creep deformation mechanism, including conducting creep fracture tests on materials at different stress levels at different test temperatures; establishing a functional relationship between steady-state / minimum creep strain rate and stress σ and temperature T based on the creep fracture tests, denoted as Equation 1; obtaining the relationship between creep strain ε and creep time t based on Equation 1, denoted as Equation 2; and obtaining the creep fracture time t based on Equation 2. r The calculation formula is given; considering the influence of grain size, the corrected creep rupture time t is obtained. r Using the strain at 90% of the fracture life as the creep ductility-modified ductility exhaustion method, the creep strain ε is obtained. 0.9 The relationship between and ; according to t r ′、ε 0.9 The expression is used to obtain the creep life calculation formula. The high-temperature creep life prediction method of this invention considers the effects of different creep mechanisms, size effects, and plastic deformation during creep fracture, and can accurately predict the life of materials under high-temperature creep.

Description

Technical Field

[0001] This invention relates to the field of life prediction, and in particular to a method for predicting high-temperature creep life based on creep deformation mechanism. Background Technology

[0002] In fields such as nuclear power, aerospace, and chemical engineering, equipment typically operates under high-temperature conditions. Creep at these temperatures can lead to material failure, making creep life a crucial performance characteristic of high-temperature materials, and its prediction essential. Currently, widely used parametric methods such as Larson-Miller (LM) and Orr-Sherby-Dorn (OSD) extrapolate creep life based on short-term creep data, but these methods cannot predict creep life under different dominant creep mechanisms. Furthermore, true stress-strain models based on deformation mechanisms do not consider the influence of grain size, resulting in low accuracy in material life prediction. Traditional ductility exhaustion methods use fracture strain as creep ductility to establish the relationship between creep ductility and the minimum creep rate, without considering the impact of plastic deformation caused by necking before fracture on fracture strain. Summary of the Invention

[0003] To address the shortcomings of the existing technologies, this invention provides a creep lifetime prediction method based on creep deformation mechanism. It uses the strain at 90% lifetime as a creep ductility correction ductility depletion method and considers the influence of grain size, which can accurately predict the creep lifetime of materials dominated by multiple creep mechanisms.

[0004] To achieve the above objectives, the present invention provides a method for predicting high-temperature creep life considering the effects of different creep mechanisms, comprising the following steps:

[0005] S1: The material is subjected to creep rupture tests at different stress levels under different test temperatures;

[0006] S2: Based on the results of the creep rupture experiment, establish the steady-state / minimum creep strain rate of the material. The functional relationship between stress σ and temperature T;

[0007] S3: Based on the above steady-state / minimum creep strain rate The functional relationship between stress σ and temperature T is used to obtain the functional relationship between creep strain ε and creep time t.

[0008] S4: Based on the above functional relationship between creep strain ε and creep time t, the creep fracture time t is obtained. r The calculation formula;

[0009] S5: Considering the size effect, the corrected creep rupture time t is obtained. r The calculation formula for ′;

[0010] S6: Using the strain at 90% of the fracture life as the ductility exhaustion method for creep ductility, the creep ductility ε is obtained. 0.9 With steady-state / minimum creep strain rate The functional relationship;

[0011] S7: Based on the steady-state / minimum creep strain rate The functional relationship between stress σ and temperature T, and the corrected creep rupture time t r The calculation formula for ', the creep ductility ε 0.9 With steady-state / minimum creep strain rate From the functional relationship, the creep fracture time t is obtained. r The creep life of the material is predicted by using the functional relationship between stress σ and temperature T.

[0012] Further, S2 includes:

[0013] S21: Creep strain rate under different creep deformation mechanisms, using Norton's power-law equation. The functional relationship between stress σ and temperature T is expressed as:

[0014]

[0015] In equation (1), i = s, g, c; s represents grain boundary slip creep mechanism, g represents dislocation slip creep mechanism, and c represents dislocation climb creep mechanism (the subscript i in this paper all have this meaning and will not be repeated); A represents material constant, n represents stress exponent; Q represents creep activation energy; R represents molar gas constant, R = 8.314 J / (K·mol).

[0016] S22: Using the BMD equation, rewrite equation (1) and use the tensile strength σ T Or, the elastic modulus E can be used to normalize the stress σ (for ease of writing, only the tensile strength σ is listed). T The formula for regularizing stress, but σ in the formula... T (E can be used to replace all of them), rewritten creep strain rate The functional relationship between stress σ and temperature T is expressed as:

[0017]

[0018] In equation (2), A0 represents the material constant; D0 represents the diffusion coefficient; k represents the Boltzmann constant; E represents the elastic modulus of the material at the test temperature; and b represents the Burgers vector.

[0019] S23: The creep rates of each deformation mechanism are linearly summed to obtain the steady-state / minimum creep rate. for:

[0020]

[0021] In equation (3), A i ′=A i0 D i0 (b / k)

[0022] S24: Introducing the true stress-strain relationship:

[0023] σ=σ0exp(ε) (4-a)

[0024] ε=ln(1+e) (4-b)

[0025] In equations (4-a) and (4-b), σ0 represents the nominal engineering stress, and e represents the engineering strain.

[0026] S25: Consider the dislocation propagation effect caused by the dislocation climb creep deformation mechanism:

[0027]

[0028] In equation (5), when i = s, g, M i =0; M represents the dislocation increment coefficient.

[0029] S26: Based on equations (4-a) and (5), and using Taylor series expansion, we obtain:

[0030]

[0031] In equation (6),

[0032] Further, S3 includes:

[0033] S31: Integrating equation (6) yields the creep strain ε in the second stage of creep. ss :

[0034]

[0035] S32: According to the transient creep calculation formula under the grain boundary slip creep deformation mechanism, the functional relationship between creep strain ε and creep time t is expressed as:

[0036]

[0037]

[0038] In equation (8), ε e0 Indicates the initial elastoplastic strain; This represents the creep strain in the first stage. β represents the material constant, and H represents the work hardening coefficient of the grain boundary slip mechanism; (When i = s), This indicates the creep time in the first stage.

[0039] Furthermore, based on the aforementioned functional relationship between creep strain ε and creep time t, when the creep time t is very large, exp(-t / t) T The initial elastic-plastic strain ε in equation (8) tends to 0 and can be ignored. e0 The creep rupture time t in S4 r The formula for calculation is:

[0040]

[0041] In equation (9), ε r This represents the creep fracture strain.

[0042] Furthermore, in S5, considering grain size, the corrected fracture time t r The calculation formula is:

[0043]

[0044] In equation (10), p is the grain size index.

[0045] Furthermore, in S6, the strain ε at 90% of the fracture life is used. 0.9 As creep failure strain ε r Establish ε 0.9 With steady-state / minimum creep strain rate The functional relationship is:

[0046]

[0047] Furthermore, according to the above equations (6), (10), and (11), the creep rupture time tr in S7 is... ′ The functional relationship between stress σ and temperature T is:

[0048]

[0049] The creep life prediction method based on creep deformation mechanism of the present invention has the following advantages:

[0050] 1) The strain at 90% of the fracture life is used as the fracture strain modified ductility exhaustion method model, enabling this method to accurately predict the creep life of materials. Furthermore, it is applicable to creep life under different temperature and stress conditions, considering different creep mechanisms as the dominant factors.

[0051] 2) Consider the influence of grain size to improve prediction accuracy. Attached Figure Description

[0052] Figure 1 This is a flowchart of the creep life prediction method of the present invention;

[0053] Figure 2 A graph showing the fitting results of obtaining the stress index according to an embodiment of the present invention;

[0054] Figure 3 A fitting result diagram for obtaining the creep activation energy according to an embodiment of the present invention;

[0055] Figure 4 To determine the strain ε at 90% fracture life according to an embodiment of the present invention 0.9 As creep fracture strain ε r The fitting results of the constant of the ductility exhaustion method are shown in the figure.

[0056] Figure 5 This is a graph showing the predicted lifetime results for an example of the present invention. Detailed Implementation

[0057] The invention will now be described in further detail with reference to the accompanying drawings and specific examples, so as to better understand the functions and features of the invention.

[0058] like Figure 1 As shown, the creep life prediction method of the present invention includes the following steps:

[0059] S1: At different test temperatures, the example section selects two temperature points, 800℃ and 850℃, to conduct creep rupture tests on SA508-Ⅲ steel at stress levels of 15-30MPa.

[0060] S2: Based on the results of the creep rupture experiment, the steady-state / minimum creep strain rate of the material is established by fitting the material constants using double logarithmic coordinates. The functional relationship between stress σ and temperature T.

[0061] S3: Based on the above steady-state / minimum creep strain rate The functional relationship between stress σ and temperature T is used to obtain the functional relationship between creep strain ε and creep time t.

[0062] S4: Based on the above functional relationship between creep strain ε and creep time t, the creep fracture time t is obtained. r The calculation formula.

[0063] S5: Considering the size effect, the above calculation formula is modified.

[0064] S6: Using the strain at 90% of the fracture life as the ductility exhaustion method for creep ductility, the creep ductility ε is obtained. 0.9 With steady-state / minimum creep strain rate The functional relationship.

[0065] S7: Based on the steady-state / minimum creep strain rate The functional relationship between stress σ and temperature T, the functional relationship between creep strain ε and creep time t, and the creep fracture strain ε r With steady-state / minimum creep strain rate From the functional relationship, the creep fracture time t is obtained. r The creep life of the material is predicted by using the functional relationship between stress σ and temperature T.

[0066] The following is a detailed description of S2-S6:

[0067] S2 establishes steady-state / minimum creep strain rate The functional relationships between stress σ and temperature T include:

[0068] First, using the Norton power-law equation, the creep strain rate under different creep deformation mechanisms is analyzed. The functional relationship between stress σ and temperature T is expressed as:

[0069]

[0070] In equation (1), i = s, g, c; s represents grain boundary slip creep mechanism, g represents dislocation slip creep mechanism, and c represents dislocation climb creep mechanism (the subscript i in the following text all have this meaning and will not be elaborated further); A represents the material constant, n represents the stress exponent; Q represents the creep activation energy; R represents the molar gas constant, R = 8.314 J / (K·mol). The creep rate versus stress double logarithmic coordinate curve is shown below. Figure 2 The stress exponent n is obtained by fitting the data as shown.

[0071] In the example, the creep rupture test described in S1 was performed on SA508-Ⅲ steel. Combined with creep test data of SA508-Ⅲ steel at 900℃ and 1000℃ from the literature, the stress exponents at 800℃, 850℃, 900℃, and 1000℃ were fitted to be 3.56, 3.36, 3.22, and 3.40, respectively. Based on the parameter table corresponding to different creep mechanisms, it can be concluded that the creep mechanism of the above material at 800-1000℃ is controlled by a single dislocation slip mechanism. Taking n... g =3.

[0072] Next, the BMD equation is used to rewrite equation (1), and the stress is regularized. The tensile strength σ is then used. T Or, regularize the stress σ using the elastic modulus E, and then rewrite the creep strain rate. The functional relationship between stress σ and temperature T is expressed as:

[0073]

[0074] In equation (2), A0 represents the material constant; D0 represents the diffusion coefficient; k represents the Boltzmann constant; E represents the elastic modulus of the material at the test temperature; and b represents the Burgers vector. In the example, the elastic modulus E is used to regularize the stress σ, and the creep strain rate is rewritten as follows. The functional relationship between stress σ and temperature T is expressed as:

[0075] Then, the creep rates of each deformation mechanism are linearly accumulated. In the example, under temperature and stress conditions, the dominant creep mechanism is dislocation slip, thus obtaining the steady-state / minimum creep rate. for:

[0076]

[0077] In equation (3), A i ′=A i0 D i0 (b / k)

[0078] In the example, In the formula A g ′=A g0 D g0 (b / k).

[0079] Then, considering the change in stress and strain caused by the change in specimen shape during the creep test, the true stress-strain relationship is introduced:

[0080] σ=σ0exp(ε) (4-a)

[0081] ε=ln(1+e) (4-b)

[0082] In equations (4-a) and (4-b), σ0 represents the nominal engineering stress, and e represents the engineering strain.

[0083] Then, the dislocation propagation effect caused by the dislocation climb creep deformation mechanism is considered:

[0084]

[0085] In equation (5), when i = s, g, M i =0; M represents the dislocation increment coefficient. In the example, i = g, therefore

[0086] Finally, by substituting equation (4-a) into equation (5) and expanding the exponential term using Taylor series, we obtain the following by taking the first two terms of the expansion:

[0087]

[0088] In equation (6),

[0089] In this example In the formula,

[0090] S3 establishes the functional relationship between creep strain ε and creep time t, including:

[0091] First, integrate equation (6) to obtain the creep strain ε in the second stage of creep. ss :

[0092]

[0093] In the example

[0094] Next, based on the transient creep calculation formula under the grain boundary slip creep deformation mechanism, the functional relationship between creep strain ε and creep time t is expressed as:

[0095]

[0096] In equation (8), ε e0 Indicates the initial elastoplastic strain; This represents the creep strain in the first stage. β represents the material constant, and H represents the work hardening coefficient of the grain boundary slip mechanism; (When i = s), This represents the creep time in the first stage. In this example, the material is controlled by a single dislocation slip mechanism, therefore...

[0097] In S4, because the creep test time of the material is very long, exp(-t / t) T The initial elastic-plastic strain ε in equation (8) tends to 0 and can be ignored. e0 The simplified equation (8) yields the creep rupture time t. r The formula for calculation is:

[0098]

[0099] In equation (9), ε r This represents the creep fracture strain.

[0100] In the example

[0101] In S5, consider the size effect:

[0102]

[0103] In equation (10), p is the grain size index. In this example...

[0104] In S6, the strain ε at 90% of the fracture life is used. 0.9 As creep fracture strain ε r Establish ε 0.9 With steady-state / minimum creep strain rate The functional relationship is:

[0105]

[0106] In S7, according to equations (6), (10), and (11), the creep rupture time t r The functional relationship between ′ and stress σ and temperature T is:

[0107]

[0108] In this example,

[0109]

[0110] The creep life prediction method of this invention is used to predict the creep life of SA508-Ⅲ steel at 800℃ and 900℃. First, based on the creep rupture test described in S1, and combined with creep test data of SA508-Ⅲ steel at 900℃ and 1000℃ from the literature, the stress exponents at 800℃, 850℃, 900℃, and 1000℃ are fitted, as follows: Figure 2 As shown, the values ​​are 3.56, 3.36, 3.22, and 3.40, respectively. Based on the parameter table corresponding to different creep mechanisms, it can be concluded that the above materials at 800-1000℃ are controlled by a single dislocation slip mechanism, therefore the stress exponent n... g =3, grain size index p g =0, and then, based on the experimental data at 800℃, 850℃, 900℃, and 1000℃, it can be fitted that, in the temperature range of 800-1000℃, the creep activation energy is from Figure 3 The fitting yielded Q g =61638J / mol, A g ′=A g = 24292. Based on the creep curves at 800℃ and 850℃, the strain ε at 90% of the fracture life is... 0.9 As creep fracture strain ε r In the ductile exhaustion method, based on creep rupture test data at 800℃ and 850℃, the material constant is determined by... Figure 4 Through fitting, C = 44 and m = 0.14 were obtained in this example. The creep life of SA508-Ⅲ steel at 800℃ with stresses of 14MPa and 26MPa, and at 900℃ with stresses of 18MPa, 22MPa, and 30MPa, was predicted. The predicted results were compared with the experimental results, as follows: Figure 5 As shown.

[0111] from Figure 5 It can be seen that the predicted life accuracy is within 2.5 times the error band, the experimental results are close to the predicted results, and the predicted results are conservative. Therefore, the creep life prediction model shown in this invention can predict the creep life of SA508-Ⅲ steel at different stress levels at 800℃ and 900℃.

Claims

1. A method for predicting high-temperature creep life based on a creep deformation mechanism, characterized by, The method comprises the following steps: S1: performing creep rupture tests on the material at different stress levels at different test temperatures; S2: from the results of the creep rupture test, establishing a function relating the steady state / minimum creep strain rate of the material to the stress and temperature ​​ S3: the steady state / minimal creep strain rate according to the above function of stress , temperature ; and creep strain as a function of time ; S4: The creep strain according to the above With the creep time The function relationship of the change, the creep rupture time The calculation formula, S5: considering the size effect, the creep rupture time is obtained after correction of the calculation formula, S6: Adopting the strain at 90% of the breaking life As a ductility exhaustion method of creep ductility, the creep ductility The function relationship of the steady state / minimum creep strain rate as a function of S7: a function of the steady state / minimum creep strain rate and stress , temperature , the modified creep rupture time , the creep ductility and steady state / minimum creep strain rate , the creep rupture time and stress , temperature , the creep life of the material; In the step S5, the influence of grain size is considered; The ductility depletion method calculation formula established in the step S6 is: (11)。 2. The high-temperature creep life prediction method based on the creep deformation mechanism according to claim 1, characterized by, The step S2 specifically comprises the following steps: S21: The function relationship between the creep strain rate and stress, temperature under different creep deformation mechanisms is expressed in the form of Norton power law equation and stress , temperature is expressed as: (1) in formula (1), ; represents a grain boundary sliding creep mechanism, represents a dislocation sliding creep mechanism, represents a dislocation climbing creep mechanism; represents a material constant, represents a stress exponent; represents a creep activation energy; represents a molar gas constant, ; S22: rewrite equation (1) using BMD equation, and use tensile strength or elastic modulus regularized stress , the rewritten creep strain rate and stress , temperature The functional relationship between them is expressed as: (2) in formula (2), represents a material constant; represents a diffusion coefficient; represents the Boltzmann constant; represents the elastic modulus of the material at the test temperature; represents the Burgers vector; S23: Linearly accumulate the creep rate of each deformation mechanism to obtain the steady state / minimum creep rate is: (3) in formula (3), ; S24: introducing the true stress-strain relationship: (4-a) (4-b) in formula (4-a), (4-b), denotes the nominal engineering stress, denotes the engineering strain; S25: considering the dislocation multiplication effect caused by the dislocation climb creep deformation mechanism: (5) In formula (5), when ; ; represents a dislocation multiplication coefficient; S26: according to formula (4-a), formula (5), and using Taylor series expansion to obtain: (6) In formula (6), .

3. The high-temperature creep life prediction method based on the creep deformation mechanism according to claim 2, characterized by, The step S3 comprises the following steps: S31 : Integrating equation (6) gives the creep strain in the second stage of creep : (7) S32: The creep strain is calculated according to the transient creep calculation formula under the grain boundary sliding creep deformation mechanism with the creep time The functional relationship of the change is represented as: (8) In formula (8), represents the initial elastic-plastic strain; represents the first stage creep strain, , represents a material constant, represents a grain boundary sliding mechanism work hardening coefficient; when , , represents the first stage creep time.

4. The high-temperature creep life prediction method based on the creep deformation mechanism according to claim 3, characterized by, The step S4 establishes the creep rupture time The calculation formula is: (9)。 5. The high-temperature creep life prediction method based on the creep deformation mechanism according to claim 4, characterized by, The creep rupture time established by the step S5 The calculation formula is: specifically: (10) In formula (10), , , is a grain size exponent.

6. The high-temperature creep life prediction method based on the creep deformation mechanism according to claim 5, characterized by, The step S7 establishes the creep rupture time with stress , temperature The function relationship is: (12)。

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