1Department of Production Engineering, National Institute of Technology, Tiruchirappalli-620 015, Tamil Nadu, India.
2Welding Research Institutem, 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. For a long time the T22 grade has been in use world wide in high capacity power stations with excellent operating behavior. T22 is low alloyed steel contains 2.25% Cr and addition of 1% Mo. 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. Thermal shock loading of operating pressure equipment is a common occurrence, particularly in thermal power stations. These tubes subjected to the temperature of 800°C and producing steam at a temperature of 540°C and a pressure of 320 Kg/cm2. Apart from the thermal fatigue failures, short–term and long-term over heating failures will also occur. A short term over heating failure is one in which a single incident or a small number of incidents exposes the component to an excessively high temperature (hundreds of degrees above normal), to the point where deformation or yielding occurs. Overheating results from abnormal conditions such as loss of coolant flow and excessive temperature excursion. In this paper a laboratory simulation for reproducing short term over heating phenomenon were developed. The tests were conducted in a TIG welded tube which was passed in non-destructive tests. The tube is subjected to thermal cycles from 800°C to room temperature. Oxy-acetylene 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. After 4 cycles bursting of the tube in the weld region was occurred with great sound. A knife edge rupture failure is identified in the weld region. The length and width of the crack opening is about 6.5 mm and 8.5 mm respectively. The tubes are then sectioned and subjected to optical microscopy. Cracks and cavities are observed in the failed weld tube. The causes of the failures were thoroughly investigated using Scanning Electron Microscope (SEM). This study revealed that short–term over heating is the cause for the failure of the tube.
Short–term over heating, T-22 weld tube, Localised heating, Oxy-acetylene torch, knife edge rupture failure, Micro cracks and cavities, Failure analysis
