A method for predicting uniaxial fatigue during hot aging of cast stainless steel based on thermoelectric potential

Abstract

This invention relates to the field of fatigue life prediction after hot aging of cast stainless steel, and uses thermoelectric potential detection parameters to predict the uniaxial low-cycle fatigue life of cast stainless steel after hot aging. It includes the following steps: Step A, performing strain measurement or finite element calculation on the critical parts of the cast stainless steel structure in service to obtain the strain amplitude ε of the critical parts. a Step B, strain amplitude ε at the critical location. a Make corrections and calculate the equivalent amplitude ε. ea Step C: Substitute the equivalent strain amplitude into the strain amplitude-fatigue life curve ε of the un-heat-aged cast stainless steel. a =f(N), calculate the uniaxial low-cycle fatigue life N after thermal aging. The advantage is that by performing non-destructive thermoelectric potential testing on cast stainless steel in the field service environment, the thermoelectric potential test parameters are used as a characterization of the degree of thermal aging of cast stainless steel, and this non-destructive parameter is introduced into the prediction of the uniaxial low-cycle fatigue life of cast stainless steel after thermal aging.

Description

Technical Field

[0001] This invention relates to the field of fatigue life prediction after hot aging of cast stainless steel, and uses thermoelectric potential detection parameters to predict the uniaxial low-cycle fatigue life of cast stainless steel after hot aging. Background Technology

[0002] In pressurized water reactor nuclear power plants, cast stainless steel, martensitic stainless steel, ferritic stainless steel, ferritic low alloy steel and nickel-based alloys will undergo thermal aging when in service for a long time in a high-temperature environment of 250-350℃. This is mainly manifested in increased hardness and strength, decreased ductility and impact strength, and reduced fracture toughness, which will lead to a decrease in the safety of the primary circuit pressure boundary.

[0003] Fatigue is also one of the aging and degradation mechanisms affecting many key components of the primary circuit pressure boundary in pressurized water reactors. Fatigue is structural failure caused by alternating stress / strain cycles induced by fluctuating loads or temperature changes. Cast stainless steel structures are subjected to alternating stress / strain cycles during service. If sufficient local fatigue damage accumulates in critical areas, fatigue cracks will appear, which will then propagate under subsequent alternating stress / strain. Low-cycle fatigue is caused by relatively high stress ranges and has fewer than approximately 10 cycles. 4 Up to 10 5 Fatigue is generally caused by the combined effects of pressure, pipe bending moment, and local thermal stress during normal operation of the power plant.

[0004] In long-term pressurized water reactors, cast stainless steel used for main coolant piping undergoes thermal aging, leading to material hardening and embrittlement. Simultaneously, thermal aging also affects the fatigue properties of cast stainless steel, both factors threatening the integrity of the reactor pressure boundary. Currently, there is limited research on the fatigue performance of cast stainless steel after thermal aging, and no unified conclusion has been reached regarding the impact of thermal aging on fatigue properties. The degree to which thermal aging affects uniaxial low-cycle fatigue life is related to the ferrite content of the cast stainless steel and the thermal aging temperature and time experienced during its service environment.

[0005] Currently, thermoelectric potential testing technology has been adopted by relevant institutions both domestically and internationally, leading to the development of related testing instruments. These instruments are used to detect changes in the microstructure and mechanical properties of metals such as cast stainless steel caused by thermal aging. Thermoelectric potential testing is a non-destructive testing method. Its basic principle utilizes the Seebeck effect: when a temperature difference ΔT exists between the two ends of a metal component, a potential difference ΔV is simultaneously generated between them. The ratio of these two (ΔV / ΔT) is called the thermoelectric potential. Therefore, the uniaxial low-cycle fatigue life of cast stainless steel after thermal aging can be predicted based on the thermoelectric potential testing parameters. Summary of the Invention

[0006] The purpose of this invention is to provide a method for predicting uniaxial low-cycle fatigue of cast stainless steel under thermal aging based on thermoelectric potential. Based on the strain amplitude-fatigue life curve of cast stainless steel before thermal aging and the strain amplitude-fatigue life data points at time points after thermal aging, which are readily available in the laboratory, the method obtains the uniaxial low-cycle fatigue life of cast stainless steel after thermal aging under field service conditions by measuring the thermoelectric potential of cast stainless steel under field service conditions based on thermoelectric potential detection parameters.

[0007] The technical solution adopted by this invention to solve the above-mentioned technical problems is as follows: a method for predicting uniaxial low-cycle fatigue of cast stainless steel during hot aging based on thermoelectric potential, comprising the following steps:

[0008] Step A: Perform strain measurement or finite element calculation on the critical parts of the cast stainless steel structure in service to obtain the strain amplitude ε of the critical parts. a ;

[0009] Step B, for the strain amplitude ε of the critical area a Make corrections and calculate the equivalent amplitude ε. ea :

[0010] ε ea =ε a (1+kPδ) (1)

[0011] Where k is the thermal aging-uniaxial low-cycle fatigue sensitivity coefficient, k>0 indicates that the fatigue life decreases after thermal aging, k<0 indicates that the fatigue life increases after thermal aging, and k=0 indicates that the fatigue life does not change after thermal aging; Pδ is the normalized thermal aging parameter Pδ, P is the Arrhenius thermal aging parameter, and δ is the ferrite content (%) of the cast stainless steel.

[0012] Step C: Substitute the equivalent strain amplitude into the strain amplitude-fatigue life curve ε of the cast stainless steel before thermal aging. a =f(N) is used to calculate the uniaxial low-cycle fatigue life N after thermal aging.

[0013] Furthermore, the calculation of the normalized thermal aging parameter Pδ in step B is based on the following relationship:

[0014] Pδ=g(TEP) (2)

[0015] TEP represents the thermoelectric potential of stainless steel.

[0016] Furthermore, the thermoelectric potential (TEP) of the stainless steel was obtained by measuring the cast stainless steel structure in field service.

[0017] Furthermore, in step B, the thermal aging-uniaxial low-cycle fatigue sensitivity coefficient k is obtained by minimizing the curve ε a =f(N) and strain amplitude-fatigue life data points εai -N i The difference between them is used to determine this.

[0018] Furthermore, the minimized curve ε a =f(N) is the strain amplitude-fatigue life curve of cast stainless steel before thermal aging, i.e., the Manson-Coffin equation:

[0019]

[0020] Where ε a Where σ' is the strain amplitude, N is the fatigue life, and σ′ is the fatigue amplitude. f ε′ is the fatigue strength coefficient, b is the fatigue strength exponent, E is the elastic modulus, and ε′ is the fatigue strength coefficient. f is the fatigue strength ductility coefficient, and c is the fatigue strength ductility index.

[0021] Furthermore, the strain amplitude-fatigue life data point ε ai -N i For strain amplitude-fatigue life data points at any arbitrary time point after hot aging of cast stainless steel:

[0022] ε ai -N i (4)

[0023] The time point after thermal aging should be selected as long as possible, and the number of data points can be a single or multiple data points at that time point, i = 1, 2, 3, ...

[0024] Furthermore, the acquisition of strain amplitude-fatigue life data points ε ai -N i Next, it is necessary to calculate the normalized thermal aging parameter P at the time points after thermal aging. i δ i .

[0025] The advantages of this invention compared to the prior art are:

[0026] (1) The present invention is a method for predicting uniaxial low-cycle fatigue of cast stainless steel based on thermoelectric potential. By performing non-destructive testing of thermoelectric potential on cast stainless steel in field service environment, the thermoelectric potential test parameters are used as the characterization of the degree of thermal aging of cast stainless steel. The non-destructive parameters are introduced into the prediction of uniaxial low-cycle fatigue life of cast stainless steel after thermal aging.

[0027] (2) The present invention is based on the thermoelectric potential-based method for predicting uniaxial low-cycle fatigue of cast stainless steel under thermal aging. It uses the strain amplitude-fatigue life curve of cast stainless steel before thermal aging and the strain amplitude-fatigue life data points at time nodes after thermal aging, which are readily available in the laboratory, as the basis. It minimizes the difference between the curve and the data points to determine the sensitivity coefficient of cast stainless steel under thermal aging-uniaxial low-cycle fatigue. By measuring the thermoelectric potential of cast stainless steel under field service conditions, it obtains the uniaxial low-cycle fatigue life of cast stainless steel after thermal aging under field service conditions based on thermoelectric potential detection parameters.

[0028] (3) The present invention provides a method for predicting uniaxial low-cycle fatigue of cast stainless steel based on thermoelectric potential. It introduces normalized thermal aging parameters to correct the strain amplitude and considers the effects of different thermal aging times, thermal aging temperatures, ferrite content and chemical composition on the thermal aging and low-cycle fatigue of cast stainless steel. It has a wide range of applications. Attached Figure Description

[0029] Figure 1 This is a flowchart of the method for predicting uniaxial low-cycle fatigue of cast stainless steel based on thermoelectric potential during hot aging, as proposed in this invention.

[0030] Figure 2 The strain amplitude-fatigue life curves and data (literature data) of cast stainless steel before and after hot aging;

[0031] Figure 3 Comparison of uniaxial low-cycle fatigue test life and predicted life before correction.

[0032] Figure 4 This is a comparison between the corrected uniaxial low-cycle fatigue test life and the predicted life. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] This invention is based on the strain amplitude-fatigue life curve of cast stainless steel before thermal aging and the strain amplitude-fatigue life data points at time points after thermal aging, which are readily available in the laboratory. It determines the thermal aging-uniaxial low-cycle fatigue sensitivity coefficient of cast stainless steel by minimizing the difference between the curve and the data points. Strain measurements or finite element calculations are performed on the critical parts of the cast stainless steel structure in service to obtain the strain amplitude of the critical parts. After measuring the thermoelectric potential of the cast stainless steel, normalized thermal aging parameters are calculated. The strain amplitude is corrected based on the aforementioned thermal aging-uniaxial low-cycle fatigue sensitivity coefficient and normalized thermal aging parameters. Finally, based on the strain amplitude-fatigue life data readily available in the laboratory and the strain amplitude and thermoelectric potential parameters measured on the cast stainless steel structure under field service conditions, the uniaxial low-cycle fatigue life of cast stainless steel after thermal aging in the field is predicted. Figure 1 As shown.

[0035] A method for predicting uniaxial low-cycle fatigue during hot aging of cast stainless steel based on thermoelectric potential includes the following steps:

[0036] Step A: Perform strain measurement or finite element calculation on the critical parts of the cast stainless steel structure in service to obtain the strain amplitude ε of the critical parts. a ;

[0037] Step B: Strain amplitude ε at the critical location a Make corrections and calculate the equivalent amplitude ε. ea :

[0038] ε ea =ε a (1+kPδ) (1)

[0039] Where k is the thermal aging-uniaxial low-cycle fatigue sensitivity coefficient, k>0 indicates that the fatigue life decreases after thermal aging, k<0 indicates that the fatigue life increases after thermal aging, and k=0 indicates that the fatigue life does not change after thermal aging; Pδ is the normalized thermal aging parameter Pδ, P is the Arrhenius thermal aging parameter, and δ is the ferrite content (%) of the cast stainless steel.

[0040] The calculation of the normalized thermal aging parameter Pδ in step B is based on the following relationship:

[0041] Pδ=g(TEP) (2)

[0042] TEP represents the thermoelectric potential of stainless steel.

[0043] The thermoelectric potential (TEP) of the stainless steel was obtained by measuring the cast stainless steel structure in service in the field.

[0044] In step B, the thermal aging-uniaxial low-cycle fatigue sensitivity coefficient k is obtained by minimizing the curve ε. a =f(N) and strain amplitude-fatigue life data points ε ai -N i The difference between them is used to determine this.

[0045] The minimized curve ε a =f(N) is the strain amplitude-fatigue life curve of cast stainless steel before thermal aging, i.e., the Manson-Coffin equation:

[0046]

[0047] Where ε a Where σ' is the strain amplitude, N is the fatigue life, and σ′ is the fatigue amplitude. f ε′ is the fatigue strength coefficient, b is the fatigue strength exponent, E is the elastic modulus, and ε′ is the fatigue strength coefficient. fis the fatigue strength ductility coefficient, and c is the fatigue strength ductility index.

[0048] The strain amplitude-fatigue life data point ε ai -N i For strain amplitude-fatigue life data points at any arbitrary time point after hot aging of cast stainless steel:

[0049] ε ai -N i (4)

[0050] The time point after thermal aging should be selected as long as possible, and the number of data points can be a single or multiple data points at that time point, i = 1, 2, 3, ...

[0051] The acquisition of strain amplitude-fatigue life data point ε ai -N i Next, it is necessary to calculate the normalized thermal aging parameter P at the time points after thermal aging. i δ i .

[0052] Step C: Substitute the equivalent strain amplitude into the strain amplitude-fatigue life curve ε of the cast stainless steel before thermal aging. a =f(N) is used to calculate the uniaxial low-cycle fatigue life N after thermal aging.

[0053] Example: Taking the effect of hot aging on the low-cycle fatigue life of cast austenitic-ferritic stainless steel as an example from the literature, the experimental results of hot aging cast stainless steel CF8M at 430℃ for 0, 300, and 1800 h are illustrated. References cited are:

[0054] Kwon JD,Woo SW,Lee Y S.Effects of thermal aging on the low cyclefatigue behavior of austenitic-ferrite duplex cast stainless steel[J].NuclearEngineering and Design,2001,206:35-44.

[0055] 1. Obtain the strain amplitude-fatigue life curve ε of cast stainless steel CF8M before thermal aging. a = f(N), which is the Manson-Coffin equation:

[0056]

[0057] in b = -0.1669, ε′ f=0.1689, c = -0.4845.

[0058] 2. Obtain the strain amplitude-fatigue life data point ε at any arbitrary time point after hot aging of cast stainless steel CF8M. ai -N i That is, after 1800 hours of thermal aging, the strain amplitude ε a Fatigue life data points at 0.3, 0.5, 0.8, 1.0, and 1.5%, such as... Figure 2 As shown.

[0059] 3. Calculate the normalized thermal aging parameter P for cast stainless steel CF8M after 1800 hours of thermal aging. i δ i P was calculated. i δ i =0.3510.

[0060] 4. By minimizing the strain amplitude-fatigue life curve ε of cast stainless steel CF8M without thermal aging a =f(N) and after 1800h of thermal aging, respectively at strain amplitude ε a Fatigue life data points ε = 0.3, 0.5, 0.8, 1.0, 1.2, and 1.5%. ai -N i The difference between them was used to determine the thermal aging-uniaxial low-cycle fatigue sensitivity coefficient k, which was calculated to be k = 2.2208.

[0061] 5. Verify the data points from 300 hours of thermal aging. Under constant temperature thermal aging conditions of 430℃, the normalized thermal aging parameter Pδ can be directly calculated to obtain Pδ = 0.2763 without conversion using the relationship Pδ = g(TEP).

[0062] 6. Calculate the equivalent amplitude ε of thermal aging for 300 hours. ea The equivalent amplitude ε was calculated using the thermal aging-uniaxial low-cycle fatigue sensitivity coefficient k and the normalized thermal aging parameter Pδ after 300 hours of thermal aging. ea =ε a (1+kPδ), obtained after 300h of thermal aging at strain amplitude ε a The equivalent amplitude ε at 0.3, 0.5, 0.8, 1.0, 1.2 and 1.5% ea = 0.4780, 0.7966, 1.2746, 1.5932, 1.9119 and 2.3899%.

[0063] 7. Calculate the uniaxial low-cycle fatigue life after thermal aging. Apply the equivalent effect amplitude ε mentioned above. ea Substitute the strain amplitude-fatigue life curves ε of the un-thermally aged data into the curves. a=f(N), and the uniaxial low-cycle fatigue life N after thermal aging is calculated respectively.

[0064] 8. Comparison of uniaxial low-cycle fatigue test life and predicted life before strain amplitude correction for cast stainless steel after hot aging. Figure 3 As shown, the comparison between the uniaxial low-cycle fatigue test life and the predicted life after strain amplitude correction is as follows: Figure 4 As shown, after correction, the errors between the predicted and tested lifetimes for most data points are within two dispersion bands and are close to the safe range. The comparison results demonstrate that the method for predicting uniaxial low-cycle fatigue life of cast stainless steel after hot aging based on thermoelectric potential detection parameters proposed in this invention can accurately predict the uniaxial low-cycle fatigue life of cast stainless steel after hot aging.

[0065] The parts of this invention not disclosed in detail are well-known technologies in the field.

[0066] Although the illustrative specific embodiments of the present invention have been described above to enable those skilled in the art to understand the invention, it should be understood that the invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the invention as defined and determined by the appended claims, and all inventions utilizing the concept of the present invention are protected.

Claims

1. A method for predicting uniaxial fatigue during hot aging of cast stainless steel based on thermoelectric potential, characterized in that, Includes the following steps: Step A: Perform strain measurement or finite element calculation on the critical parts of the cast stainless steel structure in service to obtain the strain amplitude ε of the critical parts. a ; Step B: Strain amplitude ε at the critical location a Make corrections and calculate the equivalent amplitude ε. ea : e ea =e a (1+kPδ) (1) Where k is the thermal aging-uniaxial low-cycle fatigue sensitivity coefficient, k>0 indicates that the fatigue life decreases after thermal aging, k<0 indicates that the fatigue life increases after thermal aging, and k=0 indicates that the fatigue life does not change after thermal aging; Pδ is the normalized thermal aging parameter, P is the Arrhenius thermal aging parameter, and δ is the ferrite content of cast stainless steel. The calculation of the normalized thermal aging parameter Pδ in step B is based on the following relationship: Pδ=g(TEP) (2) Where TEP is the thermoelectric potential of stainless steel; Step C: Substitute the equivalent strain amplitude into the strain amplitude-fatigue life curve ε of the cast stainless steel before thermal aging. a =f(N) is used to calculate the uniaxial low-cycle fatigue life N after thermal aging.

2. The method for predicting uniaxial fatigue of cast stainless steel based on thermoelectric potential during hot aging according to claim 1, characterized in that: The thermoelectric potential (TEP) of the stainless steel was obtained by measuring the cast stainless steel structure in service in the field.

3. The method for predicting uniaxial fatigue of cast stainless steel based on thermoelectric potential during hot aging according to claim 1, characterized in that: In step B, the thermal aging-uniaxial low-cycle fatigue sensitivity coefficient k is obtained by minimizing the curve ε. a =f(N) and strain amplitude-fatigue life data points ε ai -N i The difference between them is used to determine this.

4. The method for predicting uniaxial fatigue of cast stainless steel based on thermoelectric potential during hot aging according to claim 3, characterized in that: The minimized curve ε a =f(N) is the strain amplitude-fatigue life curve of cast stainless steel before thermal aging, i.e., the Manson-Coffin equation: Where ε a Where σ' is the strain amplitude, N is the fatigue life, and σ′ is the fatigue amplitude. f ε′ is the fatigue strength coefficient, b is the fatigue strength exponent, E is the elastic modulus, and ε′ is the fatigue strength coefficient. f is the fatigue strength ductility coefficient, and c is the fatigue strength ductility index.

5. The method for predicting uniaxial fatigue of cast stainless steel based on thermoelectric potential during hot aging according to claim 3, characterized in that: The strain amplitude-fatigue life data point ε ai -N i For strain amplitude-fatigue life data points at any arbitrary time point after hot aging of cast stainless steel: ε ai -N i (4) The time node after thermal aging should be selected as long as possible. The number of data points can be a single or multiple data points at that time node, i = 1, 2, 3, ...

6. The method for predicting uniaxial fatigue of cast stainless steel based on thermoelectric potential during hot aging according to claim 3, characterized in that: Obtain strain amplitude-fatigue life data points ε ai -N i Next, it is necessary to calculate the normalized thermal aging parameter P at the time points after thermal aging. i δ i .

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