Ratchet effect-based nuclear-grade steel long-term service aging damage evaluation method

Abstract

The invention discloses a ratchet effect-based nuclear-grade steel long-term service aging damage evaluation method, which comprises the following steps: S1, taking a plurality of samples with different aging durations, carrying out a ratchet fatigue test on each sample to obtain a curve of evolution of ratchet strain along with cycle cycles, linearly fitting the slope of a stable stage, and calculating a relative ratchet strain rate; s2, determining aging damage parameters corresponding to different aging durations according to the difference between the service lives of the samples with the aging durations being zero and the service lives of the samples with the aging durations being greater than zero; s3, establishing a relational expression between the relative ratchet strain rate and the aging damage parameter; and S4, the step S1 is repeated, the relative ratchet strain rate of the material under other working conditions is calculated, aging damage parameters are calculated according to the relational expression, and the aging damage state is evaluated. According to the evaluation method, aging damage evaluation of most types of metal materials of the nuclear power station can be achieved only through few stress cycles, time and cost are saved, evaluation efficiency is improved, the number of parameters is small, and calculation is simple.

Description

Technical Field

[0001] The invention belongs to the technical field of metal material detection and evaluation, and specifically relates to a method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect. Background Art

[0002] Nuclear power plants are power plants that use the energy released by nuclear fission or nuclear fusion reactions to generate electricity. Due to their large scale and high risk of radioactive substances, the safety of nuclear power plant equipment and facilities is particularly important. During long-term service, most nuclear-grade components experience long-term high temperatures and asymmetric stress cycles caused by temperature and pressure fluctuations. On the one hand, the combined effect of high temperature and time causes the reactor pressure vessel material to undergo thermal aging effects, causing the material to lose toughness and become brittle; on the other hand, the local structure produces a ratchet effect under asymmetric stress cycles that exceed the yield limit, and interacts with the fatigue damage caused by the stress cycle, resulting in premature failure of the material and structure. Therefore, it is necessary to establish a reliable damage assessment method to provide a reference for the design of the long life cycle of nuclear power plant equipment structures or the life extension of in-service equipment.

[0003] Commonly used metal materials for nuclear power plant equipment include low-alloy steel used in reactor pressure vessels, duplex stainless steel commonly used in nuclear-grade pipelines, and so on. In the prior art, the evaluation of aging damage of nuclear-grade steel mainly adopts static methods, such as hardness method and thermoelectric potential method. However, for low-alloy steel materials, even under long-term aging, the change of hardness or impact work is not obvious; there are also dynamic strength methods, such as fatigue test, which can be divided into stress fatigue and strain fatigue according to the load mode, focusing on the change of fatigue performance under different load modes with aging time to establish an aging damage evaluation method, but duplex stainless steel shows that soft hardening is insensitive to aging in stress-controlled or strain-controlled fatigue cycles, and the strain amplitude or stress amplitude does not change significantly after long-term thermal aging, which brings difficulties to the evaluation of aging damage of materials. Therefore, there is an urgent need for a more reliable aging damage evaluation method that can be more widely applied to nuclear-grade metal materials. Summary of the invention

[0004] The present invention is made to solve the above-mentioned problems, and aims to provide a method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A ratchet effect-based method for evaluating the long-term service aging damage of nuclear-grade steel, comprising:

[0007] Step S1: taking a plurality of nuclear-grade steel samples with different aging times and one of which has an aging time of zero, and carrying out a ratchet fatigue test on each sample under a certain working condition, obtaining a curve graph of the evolution of ratchet strain with the number of cycles, and calculating the corresponding relative ratchet strain rate RSR by linearly fitting the slope of the stable stage of the ratchet strain rate;

[0008] Step S2: according to the life difference between the samples with zero aging time and those with greater than zero aging time under the same working condition, determine the aging damage parameter ADP corresponding to each different aging time;

[0009] Step S3: establishing a relationship between the relative ratcheting strain rate RSR and the aging damage parameter ADP by the least square method;

[0010] Step S4: Repeat step S1 to calculate the relative ratchet strain rate RSR of the nuclear-grade steel under other working conditions, calculate the corresponding aging damage parameter ADP according to the relationship determined in step S3, and evaluate the aging damage state of the nuclear-grade steel under various working conditions.

[0011] Furthermore, step S1 includes the following sub-steps:

[0012] Step S1-1: setting a suitable heat aging temperature under laboratory conditions according to the actual working environment temperature of the nuclear-grade steel being tested, and performing heat aging accelerated tests with different heat aging times on multiple samples;

[0013] Step S1-2: Define the ratchet strain as the average strain per cycle, i.e., ε r =(ε min +ε max ) / 2, where ε min is the valley strain of a single cycle, ε max is the peak strain of a single cycle. Ratchet fatigue tests are performed on each sample to obtain a curve graph showing the evolution of ratchet strain with the number of cycles. The slope of the stable stage on the curve graph is determined by linear fitting to obtain the steady-state ratchet strain rate.

[0014] Step S1-3: Calculate the relative ratchet strain rate RSR of the tested nuclear grade steel at different heat aging times according to the following formula:

[0015]

[0016] Among them, ε T is the steady-state ratchet strain rate of the sample with a thermal aging time of T in the stable stage on the ratchet strain curve; ε0 is the steady-state ratchet strain rate of the sample with a thermal aging time of zero in the stable stage on the ratchet strain curve.

[0017] Preferably, the number of nodes of the heat aging time is greater than or equal to 4, that is, at least 4 samples are taken corresponding to different heat aging times.

[0018] Further, in step S2, the aging damage parameter ADP corresponding to each sample with different thermal aging time is calculated according to the following formula:

[0019]

[0020] Among them, N T is the fatigue life of the specimen with thermal aging time T; N0 is the fatigue life of the specimen with thermal aging time zero.

[0021] Furthermore, in step S3, the relational expression is in the form of:

[0022] ADP=αRSR β

[0023] Where α and β are material constants.

[0024] Furthermore, in step S3, the relative ratchet strain rate RSR and the thermal aging damage parameter ADP corresponding to different thermal aging durations are input into the computer software, and the material constants α and β are determined by fitting through the least squares method to obtain a relationship.

[0025] Furthermore, step S4 includes the following sub-steps:

[0026] Step S4-1: changing the load size, repeating step S1, and calculating the relative ratchet strain rate RSR of the nuclear-grade steel under other working conditions;

[0027] Step S4-2: Substituting the relative ratchet strain rate RSR calculated in step S4-1 into the relationship established in step S3, and further calculating the corresponding aging damage parameter ADP;

[0028] Step S4-3: Compare the aging damage parameter ADP corresponding to each operating condition with the damage safety threshold A0 of the nuclear-grade steel after aging required by the design conditions. When ADP≤A0, the nuclear-grade steel under test is judged to be safe; when ADP>A0, the nuclear-grade steel under test is judged to be unsafe.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. The ratchet effect-based nuclear-grade steel long-term service aging damage evaluation method of the present invention only requires a small number of stress cycles, i.e., a few cycles after the ratchet strain evolution enters the stable stage (second stage), which can effectively save time and cost and improve evaluation efficiency.

[0031] 2. The ratchet effect-based nuclear-grade steel long-term service aging damage evaluation method of the present invention can be widely applied to various nuclear-grade metal materials in nuclear power plants, such as low-alloy steel used in reactor pressure vessels, duplex stainless steel commonly used in nuclear-grade pipelines, etc. The inventors have found through research and analysis of commonly used metal materials in nuclear power plants that most nuclear-grade materials are more sensitive to ratchet strain (cyclic plastic strain accumulation) under stress control. Therefore, by establishing the relationship between the relative ratchet strain rate RSR and the thermal aging damage parameter ADP through ratchet strain, the aging damage evaluation of various types of metal materials in nuclear power plants can be better realized.

[0032] 3. The ratchet effect-based long-term service aging damage evaluation method for nuclear-grade steel of the present invention obtains the slope of the ratchet strain rate stable stage through linear fitting, calculates the relative ratchet strain rate, and then predicts the aging damage parameters. The process involves fewer parameters and simple calculations, and can quickly realize the evaluation of the aging damage state of materials under different load conditions, thus solving the problem of damage monitoring of long-life service components. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a flow chart of the long-term service aging damage evaluation method for nuclear-grade steel based on the ratchet effect;

[0034] Figure 2 It is a curve diagram showing the evolution of the response stress amplitude with the number of cycles under different heat aging times of the material under the condition of Δε / 2=0.5%;

[0035] Figure 3 It is a curve diagram showing the evolution of ratchet strain with cycle number corresponding to different heat aging times under 10±400MPa conditions;

[0036] Figure 4 It is a comparison chart between the calculated value and the predicted value of the thermal aging damage parameter test of the material under different working conditions;

[0037] Figure 5 This is a comparison chart of the measured values ​​and predicted values ​​of fatigue life tests of materials under different working conditions. DETAILED DESCRIPTION

[0038] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the following embodiments and the accompanying drawings specifically illustrate the long-term service aging damage evaluation method for nuclear-grade steel based on the ratchet effect of the present invention.

[0039] Example 1

[0040] like Figure 1 As shown, the present invention provides a method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect, comprising:

[0041] Step S1: taking a plurality of nuclear-grade steel samples with different aging times and one of which has an aging time of zero (i.e., original state), and carrying out a ratchet fatigue test on each sample under a certain working condition (load), obtaining a curve graph of the evolution of ratchet strain with the number of cycles, and calculating the corresponding relative ratchet strain rate RSR by linearly fitting the slope of the stable stage of the ratchet strain rate;

[0042] Step S2: according to the life difference between the samples with zero aging time and those with greater than zero aging time under the same working condition, determine the aging damage parameter ADP corresponding to each different aging time;

[0043] Step S3: establishing a relationship between the relative ratcheting strain rate RSR and the aging damage parameter ADP by the least square method;

[0044] Step S4: Repeat step S1 to calculate the relative ratchet strain rate RSR of the tested nuclear-grade steel under other working conditions, calculate the corresponding aging damage parameter ADP according to the relationship determined in step S3, and evaluate the aging damage state of the tested nuclear-grade steel under various working conditions.

[0045] Specifically, step S1 includes the following sub-steps:

[0046] Step S1-1: setting a suitable thermal aging temperature under laboratory conditions according to the actual working environment temperature of the nuclear-grade steel being tested, and performing accelerated thermal aging tests with different thermal aging times on a plurality of nuclear-grade steel samples.

[0047] Preferably, the number of heat aging time nodes is greater than or equal to 4, that is, at least 4 samples are taken corresponding to different heat aging times.

[0048] Step S1-2: Define the ratchet strain as the average strain per cycle, i.e., ε r =(ε min +ε max ) / 2, where ε min is the valley strain of a single cycle, ε max is the peak strain of a single cycle. Ratchet fatigue tests were performed on each sample to obtain a graph showing the evolution of ratchet strain with the number of cycles. The evolution of ratchet strain under various thermal aging durations can be basically divided into three stages: the initial ratchet strain rate reduction stage, then the ratchet strain rate stabilization stage, and finally the ratchet strain rate accelerated growth stage, of which the second stage, the ratchet strain rate stabilization stage, occupies the main part of the material life. With the increase of thermal aging time, the ratchet strain rate increases, that is, the corresponding slope of the second stage is larger, and a larger ratchet strain leads to a reduction in fatigue life. The slope of the stable stage on the graph showing the evolution of ratchet strain with the number of cycles is determined by linear fitting, and the steady-state ratchet strain rate can be obtained.

[0049] Step S1-3: Calculate the relative ratchet strain rate RSR of the tested nuclear grade steel at different heat aging times according to the following formula:

[0050]

[0051] Among them, ε T is the steady-state ratchet strain rate of a sample with a thermal aging time of T (T>0) at the stable stage on the ratchet strain curve; ε0 is the steady-state ratchet strain rate of a sample with a thermal aging time of zero (i.e., unaged) at the stable stage on the ratchet strain curve. The relative ratchet strain rate RSR reflects the degree of decrease in the anti-ratchet deformation ability of nuclear-grade steel materials caused by thermal aging damage.

[0052] Further, in step S2, since the fatigue life of the nuclear-grade steel ratchet is shortened due to thermal aging, the aging damage parameter ADP corresponding to each sample with different thermal aging time can be calculated according to the following formula:

[0053]

[0054] Among them, N T It is the fatigue life of the sample with thermal aging time T (T>0); N0 is the fatigue life of the sample with thermal aging time zero (i.e., no aging).

[0055] The aging damage parameter ADP reflects the degree of damage caused by aging to fatigue life. The ADP value is between 0 and 1: when ADP = 0, it means that the effect of aging on the fatigue life of the material is 0, that is, the original material; when ADP = 1, it means that the effect of aging on the fatigue life of the material reaches 100%, and the specimen cannot withstand cyclic loads.

[0056] Furthermore, in step S3, a relationship between the relative ratchet strain rate RSR and the aging damage parameter ADP is established in the following form:

[0057] ADP=αRSR β

[0058] Among them, α and β are material constants, which are related to the properties of the nuclear-grade steel material being tested. The α and β values ​​corresponding to different types of nuclear-grade steel are also different. Based on multiple sets of relative ratchet strain rate RSR and aging damage parameter ADP values, the material constants α and β in the relationship can be determined by fitting through the least squares method.

[0059] Furthermore, step S4 includes the following sub-steps:

[0060] Step S4-1: changing the load size, repeating step S1, and calculating the relative ratchet strain rate RSR of the nuclear-grade steel under other working conditions;

[0061] Step S4-2: Substituting the relative ratchet strain rate RSR calculated in step S4-1 into the relationship established in step S3, and further calculating the corresponding aging damage parameter ADP;

[0062] Step S4-3: Compare the aging damage parameter ADP corresponding to each operating condition with the damage safety threshold A0 of the nuclear-grade steel after aging required by the design conditions. When ADP≤A0, the nuclear-grade steel under test is judged to be safe; when ADP>A0, the nuclear-grade steel under test is judged to be unsafe.

[0063] Example 2

[0064] Based on Example 1, this example takes reactor pressure vessel steel as an example to illustrate the implementation process of the present invention in combination with specific conditions and parameters:

[0065] Step S1: For a certain in-service reactor pressure vessel steel, an accelerated thermal aging test is carried out at high temperature, and the thermal aging time is 0h, 1000h, 5000h and 10000h respectively. For the samples that have not been thermally aged (the thermal aging time is 0h) and other samples after thermal aging (the thermal aging time is greater than zero), a strain-controlled low-cycle fatigue test (Δε / 2=0.5%) and a stress-controlled ratchet fatigue test (working condition is 10±400MPa, where 10MPa is the average stress and 400MPa is the stress amplitude) are carried out respectively, and the curves of the response stress amplitude and the ratchet strain evolving with the number of cycles under different thermal aging times are obtained, as shown respectively. Figure 2 , 3 As shown. Figure 2 It can be seen that the cyclic soft hardening of the alloy steel is insensitive to aging, and the response strain amplitude changes under different thermal aging times are almost the same. Therefore, it is difficult to evaluate the aging damage degree of the alloy steel from the perspective of cyclic soft hardening. For stress-controlled ratchet fatigue tests, such as Figure 3 As shown in the figure, as the heat aging time increases, the ratchet strain increases, and the ratchet strain and the heat aging time show a strong correlation. Therefore, by observing the ratchet strain, the aging damage evaluation of the material can be better achieved. Based on the method of Example 1, the steady-state ratchet strain rate in the stable stage under different heat aging times is obtained by linear fitting. The relative ratchet strain rate RSR of the material under different thermal aging times was calculated and the results are shown in Table 1.

[0066] Step S2: According to the calculation formula Substitute the fatigue life N of the material under different thermal aging times T , calculate the thermal aging damage parameter ADP corresponding to different thermal aging time, the results are shown in Table 1, where N0 is N when T=T0=0 T value.

[0067] Table 1 Calculation results of relative ratchet strain rate RSR and thermal aging damage parameter ADP at different aging times

[0068]

[0069] Step S3: Input the relative ratchet strain rate RSR and thermal aging damage parameter ADP corresponding to different thermal aging durations into the computer software Origin, and determine the material constants α and β by fitting using the least squares method to obtain the relationship: ADP = 0.538RSR 0.096 .

[0070] Step S4: Perform ratchet fatigue tests on other different service conditions at the same heat aging temperature to obtain the steady-state ratchet strain rate in the stable stage. Calculate the corresponding relative ratchet strain rate RSR and substitute it into the relationship ADP=0.538RSR in step S3 0.096 The thermal aging damage parameter ADP is calculated and predicted in the process, and finally, the calculated thermal aging damage parameter ADP is compared with the damage safety threshold A0 of the low alloy steel after aging required under the design conditions. In this embodiment, the design requires that the damage safety threshold A0 of the material under various load conditions is 0.4. The value of the thermal aging damage parameter ADP (i.e., "predicted ADP") obtained by substituting the relative ratchet strain rate RSR into the relationship is as follows: Figure 4 Since the predicted ADP corresponding to all the heat aging time nodes under different working conditions is greater than 0.4, the tested material will be severely damaged by heat aging at the test temperature of this embodiment, and the material is judged to be unsafe and not suitable for continued service and should be replaced in time.

[0071] To verify the above results, the fatigue life (i.e., "test life") actually measured by the ratchet fatigue test in step S4 is used. Further calculate the ADP value corresponding to each aging time under each working condition (i.e., "calculate ADP"); use the thermal aging damage parameter ADP calculated in step S4 (i.e., "predict ADP"), according to Reverse calculation of fatigue life (i.e. "calculated life").

[0072] like Figure 4 As shown in the figure, the horizontal and vertical coordinate positions are determined by the "calculated ADP" and "predicted ADP" values ​​under each working condition. The coordinate points corresponding to each working condition are mostly located near the straight line with a slope of 1, and basically fall within the 1.5 times error line. Figure 5As shown in the figure, the horizontal and vertical coordinate positions are determined by the values ​​of "test life" and "calculated life" under each working condition. The coordinate points corresponding to each working condition are mostly located near the straight line with a slope of 1, and most of them fall within the 2-fold error line, which verifies the reliability and accuracy of this method.

[0073] In summary, the method for evaluating aging damage of nuclear-grade steel in long-term service based on the ratchet effect of the present invention establishes the relationship between the relative ratchet strain rate and the aging damage parameters. It only needs to test a small number of ratchet fatigue cycles (i.e., a few cycles after the ratchet strain evolution enters the second stage) to calculate the relative ratchet strain rate, and then the aging damage parameters can be predicted, and the aging damage state of the material under different load conditions can be evaluated. The problem of damage monitoring of various types of nuclear-grade steel long-life service components can be solved, and a reference basis can be provided for the design of long-life cycles of structures or the extension of the life of in-service equipment. The method has the advantages of fewer parameters and simple calculations, which can effectively save time and cost and improve evaluation efficiency. In addition, the method is not only applicable to the damage assessment of thermal aging, but also to aging damage caused by other related factors, and has a high value of promotion and application.

[0074] The above-mentioned embodiments are preferred examples of the present invention and are not intended to limit the protection scope of the present invention.

Claims

1. A method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect, characterized in that: include: Step S1: taking a plurality of nuclear-grade steel samples with different aging times and one of which has an aging time of zero, and carrying out a ratchet fatigue test on each sample under a certain working condition, obtaining a curve graph of the evolution of ratchet strain with the number of cycles, and calculating the corresponding relative ratchet strain rate RSR by linearly fitting the slope of the stable stage of the ratchet strain rate; Step S2: according to the life difference between the samples with zero aging time and those with greater than zero aging time under the same working condition, determine the aging damage parameter ADP corresponding to each different aging time; Step S3: establishing a relationship between the relative ratchet strain rate RSR and the aging damage parameter ADP by using the least square method; Step S4: Repeat step S1 to calculate the relative ratchet strain rate RSR of the nuclear-grade steel under other working conditions, calculate the corresponding aging damage parameter ADP according to the relationship determined in step S3, and evaluate the aging damage state of the nuclear-grade steel under various working conditions.

2. The method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect according to claim 1 is characterized in that: The step S1 includes the following sub-steps: Step S1-1: setting a suitable heat aging temperature under laboratory conditions according to the actual working environment temperature of the nuclear-grade steel being tested, and performing heat aging accelerated tests with different heat aging times on a plurality of the samples; Step S1-2: Define the ratchet strain as the average strain per cycle, i.e., ε r =(ε min +ε max ) / 2, where ε min is the valley strain of a single cycle, ε max is the peak strain of a single cycle. Ratchet fatigue tests are performed on each sample to obtain a curve graph showing the evolution of ratchet strain with the number of cycles. The slope of the stable stage on the curve graph is determined by linear fitting to obtain the steady-state ratchet strain rate. Step S1-3: Calculate the relative ratchet strain rate RSR of the tested nuclear grade steel at different heat aging times according to the following formula: Among them, ε T is the steady-state ratchet strain rate of the sample with a thermal aging time of T in the stable stage on the ratchet strain curve; ε0 is the steady-state ratchet strain rate of the sample with a thermal aging time of zero in the stable stage on the ratchet strain curve.

3. The method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect according to claim 2 is characterized in that: The number of nodes of the heat aging time is greater than or equal to 4, that is, at least 4 samples are taken to correspond to different heat aging times.

4. The method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect according to claim 1 is characterized in that: In step S2, the aging damage parameter ADP corresponding to each sample with different heat aging time is calculated according to the following formula: Among them, N T is the fatigue life of the specimen with thermal aging time T; N0 is the fatigue life of the specimen with thermal aging time zero.

5. The method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect according to claim 1 is characterized in that: In step S3, the relational expression is in the form of: ADP=αRSR β Where α and β are material constants.

6. The method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect according to claim 5 is characterized by: In step S3, the relative ratchet strain rate RSR and thermal aging damage parameter ADP corresponding to different thermal aging durations are input into the computer software, and the material constants α and β are determined by fitting using the least squares method to obtain the relationship.

7. The method for evaluating the long-term service aging damage of nuclear-grade steel based on the ratchet effect according to claim 1 is characterized in that: The step S4 includes the following sub-steps: Step S4-1: changing the load size, repeating step S1, and calculating the relative ratchet strain rate RSR of the nuclear-grade steel under other working conditions; Step S4-2: Substituting the relative ratchet strain rate RSR calculated in step S4-1 into the relationship established in step S3, and further calculating the corresponding aging damage parameter ADP; Step S4-3: Compare the aging damage parameter ADP corresponding to each working condition with the nuclear grade steel required by the design conditions. The damage safety threshold A0 after aging is compared. When ADP≤A0, the nuclear-grade steel under test is judged to be safe. When ADP>A0, the nuclear-grade steel being tested is judged to be unsafe.

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