| The superheater tube of Ultra-Super Critical (USC) thermal power units is easily caused creep, fatigue or creep-fatigue interaction form of failure under the condition of high temperature and variable load operating. It is a serious threat to the safe operation of the USC thermal power units. Unfortunately, researching on the crack growth of T92 superheater tube and creep failure behavior for dissimilar steel tube welded joint of T92/HR3C are still limited now. Therefore, in view of the above two problems, the crack tip stress and strain field, estimation of the plastic zone size and creep failure for dissimilar steel tube welded joint of T92/HR3C were studied under the high temperature and high pressure in a long time, and the crack growth for T92superheater tube was also investigated under the condition of variable load operating. All of them are much necessary to evaluate the running safety of the USC thermal power units.(1) The stress and strain field of crack tip was researched by numerical simulation on T92 superheater tube with a crack under the high temperature and high pressure in a long time. The results showed that under high temperature and high pressure in a long time the crack tip stress and strain was increased, the results of the finite element method solution of the plastic zone size compared with the theoretical calculation results had a certain error. It proved that the size of plastic zone had changed under the condition of high temperature and high pressure in a long time. The small yield scope was assessed that the stress intensity factor K could be used as a suitable selection fracture parameters for fatigue crack growth.(2) The T92/HR3C dissimilar steel tube welded joint without crack through numerical was analysised about the creep failure. The results showed that the maximum principal stress, equivalent stress and the stress tri-axiality located near the weld interface areas, and all increased by internal pressure increaseing. With the extension of running time, the stress in the WM regional showed different change tendency which was easy to form the high restraint stress concentration, and the stress of inner wall was decreases but the outer wall was strengthening in T92 regional. The creep cavity was easily generate and expansion at the weld interface.(3) For the mainly weakening region at WM and T92HAZ/T92 in T92/HR3C dissimilar steel tube welded joint with a crack, using high temperature fracture mechanics theory, based on the NSW/NSW-MOD model, the creep crack growth was studied in high temperature and high pressure. The results revealed that the T92HAZ/T92 was more affected by internal pressure than WM, and the creep crack growth rate was faster at the T92HAZ/T92 than WM, with the increase of the internal pressure, which can accelerate the expansion of the creep crack growth rate in two area, but the T92HAZ/T92 was more obvious.(4) For T92superheater tube with a crack, according to the actual condition of variable load operation in the USC thermal power units, using the fracture mechanics and numerical simulation, the fatigue crack growth was discussed. The results indicated that the stress intensity factor K was affected on the different boiler load, and the crack growth rate was obviously accelerated when the load fluctuated from 500 MW to 1000 MW. The variety amplitude of load, overpressure or overheating were all influenced the turning point of fatigue crack growth, but the influence of the turning point of fatigue crack growth was different.(5) Considering the holding time under the actual condition of variable load operating, the creep-fatigue crack growth was researched for the T92superheater tube with a crack. The results indicated that the extension of holding time under the same condition of variable load operating, the role of creep would lead to accelerated crack growth, and at the same holding time, the turning point of creep-fatigue crack growth was advanced under the load fluctuated from 200WM to 1000WM than the load fluctuated from 500WM to 1000WM. |
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