Failure Analysis on T 92 Steel Tube and Compared with Predicted Number of Cycles to Failure Using Coffin-manson Equation

*Department of Mechanical Engineering, Anna University of Technology Tirunelveli, Tirunelveli627 007, Tamilnadu, India **Department of Production Engineering, National Institute of Technology, Tiruchirappalli-620 015, TamilNadu, India ABSTRACT Super heater tubes are subjected to alternate heating and cooling in power plants causes crack and eventually fail. This phenomenon is termed as ‘thermal fatigue’. In this paper, a laboratory simulation for reproducing thermal fatigue phenomenon is developed to determine the number of cycles tend of failure occurring in tubes and compared with predicted number of cycles tend to failure using Coffin-Manson equation at a particular temperature. The theoretically obtained strain values are also validated with simulated results from ANSYS software for the applied temperature condition. Thermal fatigue test was conducted in the Non Destructive Test (NDT) cleared T92 base tubes. The tube was subjected to thermal cycles from 800oC (accelerated temperature) to room temperature. Oxyacetylene flame was utilised as a heating source, whereas a water bath was utilised for quenching purpose. The test is carried out until open cracks were identified. Surface cracks are identified in the base tubes after 90 cycles. In base tubes lot of longitudinal cracks and transverse cracks are identified on the heating zone. The tube is then sectioned and subjected to optical microscopy, Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Analysis (EDAX). Cracks and cavities are observed in the base tube. A comparison of predicted thermal fatigue lives with experimental results shows the deviation of 7 cycles. Bulging on the tube heated surface is identified in both the experimental and simulated results this shows that the developed model is showing good agreement with the experimental results. Comparison of the experimental and simulated strain results shows the maximum deviation of 7.5%. This study reveals that localised heating and cooling cause’s thermal fatigue, which initiates cracks in the tubes.