| With more and more new energy grid connected with power generation,resulting in power output is intermittent fluctuations,which requires thermal power steam turbine requent start-stop to stabilize the fluctuations operation.In the process of starting and stopping,the alternating thermal stress caused by the temperature fluctuation,which can aggravate the fatigue creep damage of the steam turbine rotor.Traditionally,the assessment of the life and reliability of ultra super-critical steam turbine rotor was mainly focused on the creep damage mechanism at constant temperature(maximum service temperature),and the influence of temperature on fatigue creep induced by alternating temperature is usually not considered.It is easy to cause overly conservative design and can not play its greatest potential and cause economic losses,Or the design margin is not enough to ensure the reliable operation of the unit and bring security risks.In this paper,the typical representative X12CrMoWVNbN10-1-1 steel of 9-12%Cr heat resistant steel for turbine rotor of ultra supercritical thermal power unit is studied,To explore the relationship between the macroscopic mechanical properties and microstructure evolution under the creep-fatigue loading.Full life cycle low cycle fatigue test was carried out under the conditions of typical cold start(300?),warm start(500?),hot start(550?)and stable operation(600?).By comparing the test results of constant temperature and pre-loading conditions,the temperature and holding time were obtained to promote the deterioration of mechanical properties of materials.the microstructure and precipitated phase(M23C6 and MX)of the raw material and the test sample were compared and analyzed by using optical microscope,scanning electron microscope and spectrum analyzer,The microstructure evolution and damage mechanism of the turbine rotor steel under creep fatigue loading are discussed.explore the rotor steel under creep fatigue loading microstructure evolution of structure and damage mechanism.At the same time,the fatigue crack and fracture morphology of the sample were analyzed,and the characteristics of crack initiation,the mode of crack propagation and the reasons for the failure were obtained.Test results show that: the mechanical properties under the creep fatigue loading material show cyclic softening characteristics and stress relaxation occurred in the time of strain retention.The life of the material under the pre-loading is less than that of the material under constant temperature loading,and the life of the material decreases with the increase of the temperature difference.From the microstructure analysis,it can be known that after creep fatigue at high temperature,the martensite is coarse and the grain boundary is deformed,and the brittle fracture occurs at low temperature(300?).after creep fatigue,M23C6 carbide coarsening is serious,and MX is dissolved or changed,the number of the organization is reduced.The high temperature environment exacerbates the growth of grain boundary's precipitates and dissolution of precipitates in grains,which makes the pinning effect decrease,and the microstructure is deformed,causing Plastic deformation.Through the analysis of fatigue crack and fracture surface,it can be known that the whole damage process,crack initiation and propagattion in transgranular manner.Creep damage was increased at the time of retention,and more branched cracks were found in the middle and the back of the crack.The damage process is the interactive process of fatigue and creep. |
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