1Department of Production Engineering, National Institute of Technology, Tiruchirappalli-620 015, Tamil Nadu, India.
2Welding Research Institute, Bharat Heavy Electricals Limited, Tiruchirappalli-620 014, Tamil Nadu, India.
Steels for high temperature service are a vital part of the construction of power stations. These 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. Super heater tubes are subjected to the temperature of 600°C and producing steam at a temperature of 540°C and a pressure of 160 bar. In this paper a laboratory simulation for reproducing thermal fatigue phenomenon were developed to determine the number of cycles to failure occurs in tubes. Thermal fatigue tests were conducted in the base tube under free expansion and constrained conditions. Since some super heater, tubes are subjected to constrained conditions where external constraints hinder its expansion during thermal cycle and the thermal stresses developed in this condition is more when compared to the free expansion condition. Two healthy base tubes passed in non-destructive tests are subjected to thermal cycles from 800°C (accelerating temperature) to room temperature. Oxyacetylene torch was utilized as a heating source, whereas a water bath was utilized for quenching purpose. The tests are carried out until open cracks were identified. Surface cracks are identified in the free expansion and constrained base tube after 140 and 120 cycles respectively. Dye penetrant test was carried out in the failed tubes to identify the surface cracks. In both the tubes, longitudinal and transverse cracks are identified in the heating zone. The tubes are then sectioned and subjected to optical microscopy. Cracks and cavities are observed in both tubes. This study revealed that localised heating and cooling causes thermal fatigue, which initiates cracks in the tubes. In the constrained base tube cracks are developed 20 cycles earlier when compared to the free expansion base tube. The crack length is more in the constrained base tube. This shows that the constrained condition is more severe, where the thermal stresses developed is more.
Thermal fatigue, T-23 tube, Free expansion and constrained condition, Localised heating, Oxy-acetylene flame, Transverse and Circumferential cracks, Failure analysis
