CN114297796A - Service life assessment method for dissimilar steel welding joint of power station boiler - Google Patents
Service life assessment method for dissimilar steel welding joint of power station boiler Download PDFInfo
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Abstract
The invention discloses a method for predicting the service life of a dissimilar steel welding joint of a power station boiler, which comprises the following steps: the dissimilar steel welding joint under the actual service high-temperature high-pressure variable working condition is researched through a micro damage mechanism; its damage is the result of the accumulation of two factors, namely creep damage and fatigue damage; creep damage and fatigue damage can be calculated by combining with actual complex operation conditions through a formula. When accumulated damage D of dissimilar steel welding joint of power station boilerAllWhen reaching 1, the welding joint is invalid, and the residual service life of the dissimilar steel welding joint can be calculated. The invention has the beneficial effects that: aiming at the actual service condition of the dissimilar steel welding joint of the current power station boiler, the residual service life of the dissimilar steel welding joint under the complex working condition can be accurately predicted; actual service-based micro damage mechanism of dissimilar steel welded jointResearch shows that the quantitative parameters of the carbide size are introduced, and the method has tighter physical and theoretical bases, so that the accuracy of the life prediction is greatly improved compared with the traditional method.
Description
Technical Field
The invention relates to the technical field of power station boilers, in particular to a method for predicting the service life of a dissimilar steel welding joint of a power station boiler under the actual high-temperature high-pressure complex working condition.
Background
With the stable and rapid development of economy in China, the demand on energy is continuously increased, and the increase of the power generation efficiency is a necessary trend. The main approach to increase the efficiency of thermal power plants is to increase the parameters of the working medium (steam), i.e. to increase the steam temperature and pressure. And the steam parameters are improved by matching with the thermal power metal material. Generally, austenitic stainless steel is usually used for the high-temperature part of the heating surface pipe of the utility boiler, and martensite heat-resistant steel is generally used for the part with relatively low temperature from the economic point of view. As the boiler is internally provided with a plurality of steel pipes with different component properties, various heterogeneous butt-welded joints appear. The dissimilar steel welding joint is required to bear the action of high temperature of flame and smoke of a hearth besides transmitting high-temperature and high-pressure steam, frequent deep peak regulation is normal, the dissimilar steel welding joint is mostly operated under the condition of creep deformation and fatigue multi-factor coupling of high temperature, high pressure, load fluctuation and the like for a long time, the use condition is very severe, and the dissimilar steel welding joint is a part with multiple accidents in the operation of a boiler. The process is characterized in that the process of blowing out and stopping the furnace is often caused by early failure and fracture of the dissimilar steel welding joint, in recent years, early failure and fracture accidents of the dissimilar steel welding joint occur for many times at home and abroad, and unsafe factors and great economic loss are brought to the operation of a power plant. Therefore, the service life prediction of the dissimilar steel welding joint is one of the problems which are urgently needed to be solved by the power plant at present.
For the residual life evaluation of the dissimilar steel welding joint, only creep damage is considered currently, an L-M parameter method or an isothermal line extrapolation method is mostly adopted, the service life under certain stress and temperature can be extrapolated through an accelerated creep rupture test for improving the temperature and the stress, and the service life evaluation of the dissimilar steel welding joint is mostly adopted currently. And estimating the long-term creep strength under the conditions of lower temperature and stress through an accelerated creep rupture test for improving the temperature and the stress, and further calculating the creep residual life. However, test data results show that the creep-accelerated fracture test cannot truly reflect the actual service condition due to the complex change of the microstructure during service. Because the selection of the creep test stress is far higher than the actual service condition of the dissimilar steel welded joint, the micro-structural change related to the failure has not come into existence due to short fracture time, the failure mechanism observed in a laboratory cannot reflect the actual operation condition of the dissimilar steel welded joint, and the validity of the life evaluation data is questionable.
The invention provides a life evaluation method considering actual multi-factor coupling complex working conditions such as creep deformation, fatigue and the like, and the method is based on the research of the actual service dissimilar steel welded joint micro-damage mechanism and has tighter physical and theoretical basis, so that the life prediction and actual goodness of fit are higher.
Disclosure of Invention
At present, the service life evaluation of the dissimilar steel welding joint mostly adopts an L-M parameter or an isothermal line extrapolation method, only aims at a single damage mode, and is not consistent with the actual operation working condition, and in addition, the microstructure change, namely the microstructure damage mechanism, in the actual operation process of the dissimilar steel welding joint is not considered. Aiming at the defects of the prior art, the experimental result of the creep damage mechanism research of the invention shows that the carbide plays an important role in the process of creep hole nucleation, when the carbide reaches a certain critical dimension, creep hole nucleation and growth can be generated near the carbide, and finally cracks are generated to cause failure. Therefore, the residual service life of the dissimilar steel welding joint is judged according to the size of the carbide and a combination damage accumulation formula. The method has very important practical significance for service life detection and management of the utility boiler and long-period safe operation.
The technical scheme adopted by the invention is as follows: a method for predicting the service life of a dissimilar steel welding joint of a power station boiler mainly comprises the following steps:
the method comprises the following steps: researching a dissimilar steel welded joint-microscopic damage mechanism under the actual service high-temperature high-pressure variable working condition;
step two: the damage of the dissimilar steel welding joint under the actual high-temperature high-pressure variable working condition service condition is the result of accumulation of two factors, namely creep damage and fatigue damage;
the formula is as follows: dAll＝Dc+Df
In the formula, DAllDefined as total damage when DAllWhen the numerical value of (1) is up to 1, the welding joint of the dissimilar steel is completely failed; d2DcDefined as creep-induced damage; d3DfDefined as fatigue damage caused by cyclic variations in load;
step three: creep damage was calculated using the following formula:
Dc＝k1∑t/tr
in which k is1Is a constant related to the material, t is the accumulated operation time of the dissimilar steel welding joint, trIs the time at which creep failure occurs;
after the dissimilar steel welding joint is subjected to long-term high-temperature and high-pressure service, the microstructure of the dissimilar steel welding joint is aged and damaged to a certain degree; the arrangement of carbide particles is formed on the interface, and relatively coarse carbide is formed in a low alloy steel heat affected zone; the grain-shaped carbide is continuously separated out at the interface, the size of the carbide is continuously increased along with the extension of service time, when the size of the carbide reaches a certain critical value, creep holes are formed, the creep strength of a welding joint is continuously reduced, and finally the creep holes are mutually connected to cause the failure of a dissimilar steel welding joint; the carbide size can be represented by the following formula:
d3＝k2texp(-Q/RT)
wherein d is the carbide diameter in μm; k is a radical of2Is a material dependent constant, Q is the activation energy for carbide growth, and is 272KJ mol-1R is a gas constant; when the diameter of the carbide reaches a critical value, the condition of creep failure is indicated, and creep damage can be calculated;
step four: the fatigue damage calculation formula is as follows:
Df＝∑n/N
in the formula, n is the cycle frequency of the variable working condition load; n is the cycle number when fatigue fails; n is determined by the following formula:
logN＝k3+k4logε
in the formula, k3、k4Is a constant; ε is the range of strain induced by axial secondary stress near the weld interface;
step five: through the third step and the fourth step, creep damage and fatigue damage can be calculated by combining with actual complex operation conditions.
Wherein, when power plant boiler xenogenesis steel welded joint's cumulative damage DAllAnd when the service life of the dissimilar steel welding joint reaches 1, the dissimilar steel welding joint is completely failed, and the residual service life of the dissimilar steel welding joint can be calculated.
The method for researching the microscopic damage mechanism is a brand new method, only creep damage is considered for the residual service life evaluation of the dissimilar steel welding joint at present, an L-M parameter method or an isothermal line extrapolation method is mostly adopted, the service life under certain stress and temperature can be extrapolated through an accelerated creep rupture test for improving the temperature and the stress, and the service life evaluation of the dissimilar steel welding joint at present is mostly adopted by the method. The life is calculated by estimating the long term creep strength at lower stress conditions through accelerated creep rupture tests at elevated temperatures and stresses. However, the creep-accelerated fracture test cannot truly reflect the actual service condition due to the complex change of the microstructure during service. Because the selection of the creep test stress is far higher than the actual service condition of the dissimilar steel welded joint, the micro-structural change related to the failure has not come into existence due to short fracture time, the failure mechanism observed in a laboratory cannot reflect the actual operation condition of the dissimilar steel welded joint, and the validity of the life evaluation data is questionable.
The invention has the beneficial effects that: (1) the actual service conditions of the dissimilar steel welding joint of the power station boiler at present are considered, and the residual service life of the dissimilar steel welding joint under a complex working condition (considering creep and fatigue multi-factor coupling) can be accurately predicted; (2) based on the research of the actual service heterogeneous steel welding joint micro damage mechanism, the quantitative parameters of the carbide size are introduced, and the method has tighter physical and theoretical bases, so that the accuracy of the service life prediction is greatly improved compared with the traditional method.
Drawings
FIG. 1 is a flow chart of the present invention.
Detailed Description
The key points and the protection points of the invention are as follows: the service life assessment method considering creep and fatigue multi-factor coupling actual complex working conditions is based on the research of the actual service micro-damage mechanism of the dissimilar steel welded joint, namely the creep damage is assessed through the change of the carbide diameter, and the method has tighter physical and theoretical bases, so that the service life prediction and actual goodness of fit are higher.
As shown in FIG. 1, based on the research on the actual microscopic damage mechanism of the dissimilar steel welding joint in service, the invention obtains the result that the damage of the dissimilar steel welding joint under the actual high-temperature high-pressure variable working condition service condition is accumulated by two factors, namely creep damage and fatigue damage, and the formula is as follows:
DAll＝Dc+Df
in the formula, DAllDefined as total damage when DAllWhen the numerical value of (1) is up to 1, the welding joint of the dissimilar steel is completely failed; d2DcThe method is characterized in that the method is defined as damage caused by creep, the damage of a dissimilar steel welding joint occurs under the condition of high temperature, high pressure and long-time operation, the research of a creep damage mechanism shows that the creep damage is closely related to the precipitation growth of carbide, and the size of the carbide which is a control parameter of creep failure is introduced into the damage; d3DfThe fatigue damage caused by the cyclic load change is defined, and the control parameter of the fatigue damage of the dissimilar steel welding joint is mainly the strain range caused by the cyclic load change.
Creep damage was calculated using the following formula:
Dc＝k1∑t/tr
in which k is1Is a constant related to the material, t is the accumulated operation time of the dissimilar steel welding joint, trIs the time at which creep failure occurs.
After the dissimilar steel welding joint is subjected to long-term high-temperature and high-pressure service, the microstructure of the dissimilar steel welding joint can be aged and damaged to a certain degree. The arrangement of carbide particles is formed on the interface, relatively coarse carbide, mainly M, is formed in the heat affected zone of the low alloy steel23C6. Through the existing experiments, the diameter d of the carbide has a corresponding relation with temperature and time, and the size growth of the carbide is related with the time and the temperature. The granular carbide is continuously separated out at the interface, the size of the carbide is continuously increased along with the extension of service time, when the size of the carbide reaches a certain critical value, a creep hole is formed, the creep strength of a welding joint is continuously reduced, and finally the creep hole is connected with each other to cause the failure of a dissimilar steel welding joint. Carbide growth is a significant cause of failure fracture of the welded joint. The carbide size can be represented by the following formula:
d3＝k2texp(-Q/RT)
wherein d is the carbide diameter (in μm); k is a radical of2Is the material dependent constant (. mu. m h)-1) Q is the activation energy for carbide growth, and is about 272KJ mol-1And R is a gas constant. When the diameter of the carbide reaches a critical value, the condition of creep failure is indicated, and creep damage can be calculated;
fatigue damage can be calculated by the following formula:
Df＝∑n/N
in the formula, n is the cycle frequency of the variable working condition load; n is the cycle number at which failure occurs from fatigue.
N is determined by the following formula:
logN＝k3+k4logε
wherein, k3、k4Is a constant; ε is the range of strain induced by axial secondary stress near the weld interface.
By the method and the calculation formula, creep damage and fatigue can be calculated by combining with actual complex operation conditionsStrain injury. When accumulated damage D of dissimilar steel welding joint of power station boilerAllWhen the service life of the welding joint reaches 1, the welding joint is invalid, and the residual service life of the dissimilar steel welding joint can be calculated.
Example (b): a special steel welded joint T91/TP347H of a certain station boiler reheater pipe section is used for 40000 hours under the working conditions of 600 ℃ and 25.4MPa, the special steel welded joint is subjected to the fatigue load circulation action for 5000 times under the constant strain of 0.14 percent, and the corresponding damage rate is calculated as follows:
for creep damage rate:
combining the relation of the diameters of the carbides, the critical dimension of the carbides is 1.6 mu m when the creep failure occurs, and the critical time t of the creep failure at 600 ℃ can be obtained by taking the critical dimension of the carbides near the low alloy side fusion line as the main failure judgment basisrWhen the integrated operation time of the welded joint was 40000h, 91887h, the creep integrated damage ratio D was calculatedc＝0.4353。
For fatigue damage rate:
according to the experimental result of the low cycle fatigue of the T91/TP347H dissimilar steel welding joint, the formula parameter K related to N is obtained through fitting3And K4When the dissimilar steel welded joint is subjected to fatigue damage under 0.14% constant strain, the corresponding life under 0.14% strain amplitude obtained under laboratory conditions is 17945 times when the load cycle reaches 5000 times, and the fatigue damage rate is Df＝0.2787。
The final total damage rate obtained is: dAll＝0.7140
According to the calculation, the welding joint can not reach the failure edge and can still be used continuously. Under the condition of maintaining the original working condition, the residual service life of the dissimilar steel welded joint is 16027h, and 2003 cycles.
Claims (2)
1. A method for predicting the service life of a dissimilar steel welded joint of a power station boiler is characterized by comprising the following steps: the method mainly comprises the following steps:
the method comprises the following steps: the dissimilar steel welding joint under the actual service high-temperature high-pressure variable working condition is researched through a micro damage mechanism;
step two: the study results in: the damage of the dissimilar steel welding joint under the actual high-temperature high-pressure variable working condition service condition is the result of accumulation of two factors, namely creep damage and fatigue damage; the formula is as follows:
DAll＝Dc+Df
in the formula, DAllDefined as total injury, D2DcDefined as creep-induced damage; d3DfDefined as fatigue damage caused by cyclic variations in load;
step three: creep damage was calculated using the following formula:
Dc＝k1∑t/tr
in which k is1Is a constant related to the material, t is the accumulated operation time of the dissimilar steel welding joint, trIs the time at which creep failure occurs;
after the dissimilar steel welding joint is subjected to long-term high-temperature and high-pressure service, the microstructure of the dissimilar steel welding joint is aged and damaged to a certain degree; the arrangement of carbide particles is formed on the interface, and relatively coarse carbide is formed in a low alloy steel heat affected zone; the granular carbide is continuously separated out at the interface, the size of the carbide is continuously increased along with the extension of service time, when the size of the carbide reaches a certain critical value, a creep hole is formed, the creep strength of a welding joint is continuously reduced, and finally the creep holes are mutually connected to cause the failure of a dissimilar steel welding joint; the carbide size can be represented by the following formula:
d3＝k2t exp(-Q/RT)
wherein d is the carbide diameter in μm; k is a radical of2Is a material correlation constant, Q is the activation energy of carbide growth, 272KJ mol-1, and R is a gas constant; when the diameter of the carbide reaches a critical value, the condition of creep failure is indicated, and creep damage can be calculated;
step four: fatigue damage is calculated using the following formula:
Df＝∑n/N
in the formula, n is the cycle frequency of the variable working condition load; n is the cycle number when fatigue fails; n is determined by the following formula:
log N＝k3+k4logε
in the formula, k3、k4Is a constant; ε is the range of strain induced by axial secondary stress near the weld interface;
step five: through the third step and the fourth step, creep damage and fatigue damage can be calculated by combining with actual complex operation conditions.
2. The utility boiler dissimilar steel welding joint service life prediction method according to claim 1, characterized in that: when accumulated damage D of dissimilar steel welding joint of power station boilerAllAnd when the service life of the dissimilar steel welding joint reaches 1, the dissimilar steel welding joint is completely failed, and the residual service life of the dissimilar steel welding joint can be calculated.
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| CN116050228A (en) * | 2023-03-31 | 2023-05-02 | 天津大学 | Creep life prediction method for P92 main steam pipeline welded joint |
| CN116050228B (en) * | 2023-03-31 | 2023-06-20 | 天津大学 | Creep life prediction method for P92 main steam pipeline welded joint |
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