Mathematical Modeling and Finite Element Simulation of Fatigue Failure of Boiler Heat Tube

Abstract

Boiler heat tube is the main component of power plant where the water is changed to highpressure steam, it operates under corrosive environment and high temperature. This corrosive environment and cyclic temperature fluctuation cause corrosion fatigue on boiler heater tube, and which degrade life of the component. Fatigue in corrosive environment is a situation where there is premature failure due to the action of corrosion at the surface, which shortens the time required for the crack initiation. The rate of corrosion can be affected by factors like temperature, pressure and presence of moisture that increase the rate of chemical reaction. This thesis presents corrosion rate by volumetric pit growth, crack initiation and crack propagation of boiler heat tube for power plant under cyclic thermal load. 8.72Cr-0.9Mo steel is one of the steel type which mostly used nowdays as boiler heat tube. Volumetric pit growth rate used for crack initiation and energy release rate approach (J-integral) used to find crack propagation. Temperature taken in the interval of 25℃-500℃, and it has a significant effect on rate of reaction and material properties. The results shows 906Mpa and 895Mpa thermal stress induced at crack tip at 500℃ from analytical and FEM respectively. And also energy release rate results are 33.4J mm2 and 35.43N mm2 ⁄ ⁄ from analytical and FEM respectively. The result shows the material reach fatigue limit in the range of low cycle fatigue having the value less than 104 cycles. The existence of corrosion reduce crack initiation cycles since crack initiation is speed up by the presence of corrosion pit.The results from both analytical approach and FEM have a good agreement with other literatures