Study On The Thermal Stress And Crack Growth Rates Of Boiler Header

Low-cycle fatigue is a main failure mode of power station boiler. The thermalstress caused by temperature differences is proved to be a major factor, which affectslow-cycle fatigue life. At present, domestic researches focus on stress and numericalsimulation of header joint. However, further analysis and fatigue assessment offatigue crack growth rate for header material Q245R is relatively rare. Therefore, it’sessential to accumulate data of fatigue crack growth rate at high temperature.Through finite element simulation, the thermal stress situation of boiler headerwas analyzed. Importantly, stress and strain distributions across header externalsurface in different conditions were obtained, and stress distributions along thicknessdirection were achieved. The results provide fatigue life prediction of boiler headerwith powerful data foundation and reliable load conditions.In this paper, fatigue crack growth rate experiments at room temperature and400℃high temperature through utilizing stress ratio0.05and0.5were carried out forboiler header material Q245R. The conclusion is that fatigue crack growth rate isquicker in the condition of high stress ratio and faster than that of lower temperature.The fatigue crack growth threshold experiments of Q245R piece with prefabricatedcrack at room temperature and400℃were conducted in two groups, i.e. R=0.05andR=0.5. The results show that fatigue crack growth threshold value decreases as stressratio R grows and reduces as temperature enhances.According to BS7910-2005，fatigue life prediction for boiler header was assessed,which combines simulated data and fatigue test data and applies Visual Basic software.In this fatigue assessment program, so long as original defect length and depth on theheader external surface are decided and thermal stress in the finite element simulation,fatigue crack growth threshold and parameters in Paris Law are input, crackpropagation situation of the header in the service period and remaining life can becalculated.The tensile test for Q245R at different temperature was conducted, and elasticmodulus and yield strength under different temperatures were achieved. Through thefitting of test results, both elastic modulus and yield strength grow linearly astemperature decreases. Based on Paris Law and the elastic modulus values under different temperatures, the relationship between the crack growth rate and thetemperature was analyzed.