Analysis of Thermal Cracking of an Industrial Duct Using Finite Element Simulation

Thermal stresses can play an important role in failure of mechanical parts, when the body is nonuniformly warmed with some portions of high gradient of temperature variation. Thermal cracking is a special case in which cracks are opened and grown in the body, as a result of thermal stresses. These ducts, which are used in a steel complex factory, transfer the exhaust gas of induction furnaces to the duster. The temperature of the exhaust gas is changed during each period of operation. Therefore the ducts are warmed because of high temperature of exhaust gas, but not uniformly due to precipitation of dusts inside them. After eight years of putting into operation, some peripheral or hoop cracks were appeared on the surface of the ducts. Because of low pressure of exhaust gas, thermal stresses were taken into consideration. A thermo-vision camera was used to record the distribution of temperature on the surface of the duct. It was observed that most of the duct was highly warmed, while a small zone with lower temperature was evident, under the precipitated dusts. For applying to the finite element model, an analytical expression was fitted on recorded data. Making use of ANSYS software, a static analysis was done with thermal loading to compute resulted thermal stresses distribution on the duct surface. The results of simulation are shown that: a) Maximum axial stress was greater than maximum peripheral stress; b) The maximum axial stress was located on the inner surface of duct, near the verge of low-temperature zone; c) The maximum axial stress was greater than endurance limit. Since the maximum temperature of exhaust gas varies during each operation period, these results confirm that peripheral cracks may occur under high-cycle fatigue. Furthermore, it was reported that cracks were appeared on the inner surface near the verge of precipitated dusts. This also agrees with the location of maximum axial stress in simulation results.