Abstract
This paper is concerned with the service-life assessment of 9Cr steels superheater outlet steam header subjected to realistic subcritical and future ultra-super critical flexible operating conditions. The proposed methodology is achieved via a combined program of high temperature strain-controlled fatigue tests, temperature- and time-dependent unified viscoplastic model for thermomechanical fatigue analysis, and Smith-Watson-Topper critical plane criterion for multi-axial life prediction. Samples of idealised operational transients with particular attention on the starting-up cycle is fully coupled with the computational modelling of header component for the high temperature performance assessment. The predicted results indicate that: (i) the header shell inner-bore saddles at weld regions are the critical locations that lead to earlier potential fatigue crack initiation, and (ii) the predicted lifetime under subcritical conditions correlates reasonably with the industrial experience. A steam header manufactured from MarBN operating under ultra-super critical condition is shown to have comparable life performance with the P91 header operating under subcritical condition.
Highlights
The most important challenges facing generation power plants include improving efficiency and reducing CO2 emissions, whilst maintaining safe and reliable operation.additional flexible conditions are needed in the power plant to accommodate rapid load variation of fossil-fired power plants, which will cause lifetime reduction of component due to alternating stresses during transient processes [1, 2]
The first part of this paper introduces the MarBN steel, low cycle fatigue (LCF) and creep-fatigue interaction (CFI) experimental results, followed by an anisothermal temperature- and time-dependent viscoplasticity material modelling framework including the material parameter identification and calibration
The ultra-super critical (USC) condition for MarBN steel with the identified parameters is first adopted in order to study the viscoplastic deformation behavior and to identify the failure locations of the superheater header
Summary
The most important challenges facing generation power plants include improving efficiency and reducing CO2 emissions, whilst maintaining safe and reliable operation.additional flexible conditions are needed in the power plant to accommodate rapid load variation of fossil-fired power plants, which will cause lifetime reduction of component due to alternating stresses during transient processes [1, 2]. The most important challenges facing generation power plants include improving efficiency and reducing CO2 emissions, whilst maintaining safe and reliable operation. The thermodynamic efficiency of fossil-fired power plants can be improved by increasing the pressure and boiler outlet temperature, i.e. operating above 600°C is termed as ultra-super critical (USC) [3]. Key issues still exist, in relation to life prediction to guide the scope of periodic outage inspections of critical high temperature components (typically fabricated from a range of ferritic steels and 9Cr steels, such as P91). From a material perspective the challenges include: (i) extrapolation to more realistic plant operations resulting in creep-fatigue interaction (CFI) deformation, and (ii) thermomechanical fatigue (TMF) due to the combination of flexible operation and complex thick-wall component geometries [5,6,7,8,9]
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