Prediction method for high-temperature creep endurance life

Abstract

The invention provides a method for predicting high-temperature creep endurance life, which comprises the following steps of: respectively processing and analyzing creep test data of a target material in a high-stress state and a low-stress state by setting a boundary load value; the creep threshold value and the endurance life in the low-stress state are extrapolated by using the minimum creep rate and the endurance life in the high-stress state and the minimum creep rate in the partial low-stress state, and compared with the prior art, the accuracy of the obtained low-stress endurance life is higher. According to the method, an ultra-long low-stress endurance test is not needed, so that the test period and the test cost are greatly reduced.

Description

Technical Field

[0001] The invention relates to the technical field of high temperature mechanical testing, and in particular to a method for predicting high temperature creep endurance life. Background Art

[0002] High-temperature service components are generally designed based on service life. For example, turbine and boiler components are designed for a service life of 30 years because endurance is a key performance indicator. For components that limit creep deformation during service, creep rate is an important reference data.

[0003] Usually at a certain temperature, the stress σ and the endurance life t u The double logarithmic coordinates are used to organize the rupture strength test data, and the stress σ and creep rate The double logarithmic coordinates are used to organize the creep rate test data. The curve relationship under high stress and short time is usually a straight line, but the curve under low stress and long time deviates from the original straight line relationship, resulting in the life predicted by extrapolating high stress data being higher than the actual low stress endurance life, and the creep rate predicted by extrapolating high stress data being lower than the actual creep rate.

[0004] The reasons why the material deviates from the original linear relationship after long-term service under low stress may be as follows: the creep and damage mechanisms of high stress and low stress have changed. Under high stress, external cross-section damage is the main form of damage, while under low stress, internal cross-section damage such as voids, microcracks and other defects are the main forms of damage. As the stress decreases, the contribution of diffusion creep increases, resulting in an accelerated creep rate. In addition, the degradation of the material's organization under high temperature for a long time leads to a weakening of the endurance strength, an increase in the corresponding creep rate, and a shortened fracture life.

[0005] Since the low-stress creep rupture time takes several years or even more than a decade, the test cycle is extremely long and difficult to obtain and use to evaluate the endurance performance and remaining life. Therefore, it is hoped to use high-stress creep data to extrapolate the required long-term data as design data. However, there are obvious inflection points in the endurance strength curve and creep rate curve during long-term service at high temperature. Large-scale extrapolation will lead to erroneous results and pose a safety hazard. Summary of the invention

[0006] In view of the above-mentioned defects, the present invention provides a method for predicting high-temperature creep rupture life to solve the problem of low derivation accuracy in the prior art.

[0007] The present invention provides a method for predicting high temperature creep rupture life, comprising the following steps:

[0008] Set the delimiting load value σ C , when the test load σ>σ CWhen σ<σ C When , σ belongs to the low load range, and the target material is in a low stress state;

[0009] The target material is processed into N1 specimens with the same nominal size, the specimen number of the specimen is i, i=1, 2...N1, N1 load values ​​are selected in the high load range, and creep tests are performed on the N1 specimens under different high loads until the specimen breaks; the failure time of the specimen is recorded as t ui , the minimum creep rate obtained is recorded as

[0010] The failure time t of the target material under high stress state ui and minimum creep rate The power function is fitted according to formula (1):

[0011]

[0012] Obtain a first fitting coefficient B and a first fitting index n;

[0013] The target material is processed into N2 specimens with the same nominal size, the specimen number of the specimen is j, j = 1, 2 ... N2, N2 load values ​​are selected within the low load range, and creep tests are performed on the N2 specimens under different low loads until the specimen enters the second stage of creep; the load borne by the specimen is recorded as σ j , the minimum creep rate obtained is recorded as

[0014] The minimum creep rate of the target material under low stress state and load σ j The power function is fitted according to formula (2):

[0015]

[0016] Obtain a second fitting coefficient A and a second fitting index m;

[0017] Substituting equation (2) into equation (1), we can obtain the endurance life t of the target material under low stress state: uj The calculation formula is:

[0018] t uj =A n Bσ j mn Formula (3).

[0019] Preferably, the target material is subjected to a critical load value σ obtained by test or formula. C The minimum creep rate corresponding to and failure time tuC , and t uC Participate in the function fitting of equation (1) and equation (2).

[0020] Preferably, the critical load value σ C The value range is Among them, R p0.2 is the yield strength of the target material at the test temperature T.

[0021] Preferably, N1≥4, N2≥3.

[0022] Preferably, the step of: calculating the load corresponding to when the creep deformation reaches the rated value within the design life according to the design requirements and formula (2), recorded as the creep threshold value σ th .

[0023] The invention is beneficial in that the boundary load value is set, the creep test data of the target material under high and low stress states are processed and analyzed respectively, and the minimum creep rate and endurance life under high stress state and the minimum creep rate under some low stress states are used to extrapolate the creep threshold value and endurance life under low stress state. Compared with the prior art, the low stress endurance life obtained by the invention has higher accuracy. The invention does not require an ultra-long low stress endurance test, which greatly reduces the test cycle and test cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the relationship curve between the creep stress, the minimum creep rate and the endurance life adopted in the present invention;

[0025] Figure 2 The relationship curve between the endurance life and the minimum creep rate obtained in the embodiment of the present invention;

[0026] Figure 3 The relationship curve between creep stress and minimum creep rate obtained in the embodiment of the present invention;

[0027] Figure 4 This is a curve showing the relationship between creep stress and endurance life obtained in an embodiment of the present invention. DETAILED DESCRIPTION

[0028] The specific embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings. These embodiments are only used to illustrate the present invention, but not to limit the present invention.

[0029] The present invention provides a method for predicting high temperature creep rupture life, comprising the following steps:

[0030] S1. Obtain the yield strength R of the target material at the test temperature T p0.2 , set the boundary load value σC , σ C The value range is It can be selected according to the target material. C When σ<σ C When , σ belongs to the low load range and the target material is in a low stress state.

[0031] S2. Process the target material into N1 specimens with the same nominal size. Each specimen has a specimen number of i, i=1, 2, ... N1. Install the N1 specimens on the creep testing machine, clamp a high-temperature extensometer to measure the strain, select N1 load values ​​in the high load range, and conduct creep tests on the N1 specimens at different high loads until the specimen breaks. The load borne by the specimen is recorded as σ i , the failure time is denoted as t ui , the minimum creep rate obtained is recorded as Preferably, N1≥4, and can be 4 or 5.

[0032] S3. Obtain the target material's critical load value σ by creep test or formula. C The minimum creep rate corresponding to and failure time t uC , and t uC Participate in the function fitting of equation (1) and equation (2).

[0033] S4. The failure time t of the target material under high stress state ui and minimum creep rate and failure time t uC and minimum creep rate The power function is fitted according to formula (1):

[0034]

[0035] Get the first fitting coefficient B and the first fitting index n.

[0036] S5. Process the target material into N2 specimens with the same nominal size. Each specimen has a specimen number of j, where j = 1, 2, ... N2. Install the N2 specimens on the creep testing machine and clamp a high-temperature extensometer to measure the strain. Select N2 load values ​​in the low load range and conduct creep tests on the N2 specimens at different low loads until the specimen enters the second stage of creep. The load borne by the specimen is recorded as σ j , the minimum creep rate obtained is recorded as Preferably, N2≥3, and can be 3 or 4.

[0037] S6. Minimum creep rate of target material under low stress state and load σ j , and failure time t uC and minimum creep rate The power function is fitted according to formula (2):

[0038]

[0039] Obtain a second fitting coefficient A and a second fitting index m.

[0040] S7. Substitute equation (2) into equation (1) to obtain the endurance life t of the target material under low stress state uj The calculation formula is:

[0041] t uj =A n Bσ j mn Formula (3).

[0042] S8. According to the design requirements and formula (2), calculate the corresponding load when the creep deformation reaches the rated value within the design life, which is recorded as the creep threshold value σ th .

[0043] The following is a specific embodiment of the present invention:

[0044] S1. In this embodiment, the target material is 9Cr-1Mo alloy, the test temperature T is 600°C, and the yield strength R p0.2 =212MPa, boundary load value σ C Select That is, 106MPa. The creep data of the target material under high and low stress states are processed and analyzed respectively, and the curve diagram used is as follows Figure 1 shown.

[0045] S2. Process the target material into 4 specimens with the same nominal size, set 4 load values ​​within the high load range, conduct creep tests under different load values ​​until the specimen breaks, and record the failure time t of each specimen. ui and minimum creep rate

[0046] S3. The target material is subjected to the critical load value σ obtained by creep test. C The corresponding failure time t uC and minimum creep rate

[0047] S4. The failure time t of the target material under high stress state ui and minimum creep rate and failure time tuC and minimum creep rate The power function is fitted according to formula (1):

[0048]

[0049] The first fitting coefficient B=0.4489 and the first fitting index n=-0.745 are obtained.

[0050] like Figure 2 As shown in FIG. 1 , according to past test data, the relationship curve between the endurance life and the minimum creep rate of the target material under high stress state can better describe the test data of the target material under low stress state. Therefore, the relationship curve between the failure time and the minimum creep rate of the target material under low stress state can use the same fitting parameters as formula (1).

[0051] S5. Process the target material into four specimens with the same nominal size and set four load values ​​σ in the low load range. j , respectively, creep tests are carried out, and there is no need to run to the rupture time for an extremely long time, only the second stage of creep is required to obtain the minimum creep rate

[0052] S6. Minimum creep rate of target material under low stress state and load σ j , and failure time t uC and minimum creep rate The power function is fitted according to formula (2):

[0053]

[0054] The second fitting coefficient A = 6.096E-18 and the second fitting index m = 5.3248 are obtained. The relationship curve is as follows Figure 3 shown.

[0055] Formula (2) is used to fit the test data of the target material under high stress state, and the high stress fitting parameter obtained is A H =2E-33,m H =12.859. Figure 3 It can be seen that the slopes of the two curves fitted based on the test data under high and low stress states are significantly different. If the data under the low stress state is extrapolated based on the slope of the curve fitted based on the test data under the high stress state, the obtained low stress minimum creep rate will be too low, which is a relatively dangerous design data.

[0056] S7. Substitute equation (2) into equation (1) to obtain the endurance life t of the target material under low stress state uj The calculation formula is:

[0057] t uj =3E12σ j -3.9669 Formula (3).

[0058] like Figure 4 As shown in the figure, if the data under low stress state is extrapolated based on the slope of the curve fitted by the test data under high stress state, the low stress endurance life obtained will be much longer than the predicted low stress endurance life obtained using formula (3). The endurance lives of the two under a load of 50 MPa are 1.57E7 and 5.4E5 respectively, with a difference of more than one order of magnitude. It can be seen that the extrapolation result using the high stress curve is unreliable.

[0059] The relationship curve between creep stress and endurance life of the target material under low stress state predicted by this embodiment has a high degree of coincidence with the low stress test data, which verifies the accuracy of the solution of the present invention.

[0060] S8. At present, some literature has the minimum creep rate The corresponding load is used as the creep threshold value of the material. The calculated creep threshold value is very small and has no practical engineering significance. In accordance with the requirement of limiting creep deformation in engineering applications, this embodiment uses the load σ corresponding to the creep deformation reaching the rated value within the design life as the creep threshold value σ th , which is more suitable for engineering design. For example, the design life of a supercritical generator set is 30 years, and the maximum creep deformation of the target material within 250,000 hours of endurance is 1%, that is, the minimum creep rate Calculate the creep threshold value according to formula (2):

[0061] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention. These improvements and substitutions should also be regarded as the scope of protection of the present invention.

Claims

1. A method for predicting high temperature creep rupture life, characterized in that: The following steps are involved: Set the delimiting load value σ C , when the test load σ>σ C When σ<σ C When , σ belongs to the low load range, and the target material is in a low stress state; The target material is processed into N1 specimens with the same nominal size, the specimen number of the specimen is i, i=1, 2...N1, N1 load values ​​are selected in the high load range, and creep tests are performed on the N1 specimens under different high loads until the specimen breaks; the failure time of the specimen is recorded as t ui , the minimum creep rate obtained is recorded as The failure time t of the target material under high stress state ui and minimum creep rate The power function is fitted according to formula (1): Obtain a first fitting coefficient B and a first fitting index n; The target material is processed into N2 specimens with the same nominal size, the specimen number of the specimen is j, j = 1, 2 ... N2, N2 load values ​​are selected within the low load range, and creep tests are performed on the N2 specimens under different low loads until the specimen enters the second stage of creep; the load borne by the specimen is recorded as σ j , the minimum creep rate obtained is recorded as The minimum creep rate of the target material under low stress state and load σ j The power function is fitted according to formula (2): Obtain a second fitting coefficient A and a second fitting index m; Substituting equation (2) into equation (1), we can obtain the endurance life t of the target material under low stress state: uj The calculation formula is: t uj = A n Bσ j mn Equation (3).

2. The prediction method according to claim 1, characterized in that: The target material is subjected to the critical load value σ obtained by test or formula C The minimum creep rate corresponding to and failure time t uC , and t uC Participate in the function fitting of equation (1) and equation (2).

3. The prediction method according to claim 1, characterized in that: The critical load value σ C The value range is Among them, R p0.2 is the yield strength of the target material at the test temperature T.

4. The prediction method according to claim 1, characterized in that: N1≥4, N2≥3.

5. The prediction method according to claim 1, characterized in that: The method also includes the following steps: according to the design requirements and formula (2), the load corresponding to the creep deformation reaching the rated value within the design life is calculated, which is recorded as the creep threshold value σ th .

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