Failure analysis of the impellers of coke plant

Premature failure of the impeller blade of coke plant has been investigated. The component functioned during the CGC gas intake cycle. The component rotates at 1480 rpm with a volumetric flow of 720 m³/min of flue gas with temperature about 200–300 °C. The failed component reveals exposed surface of a crack that extended slightly beyond the assembly weld. From macro-structural observation under-filled region in welding is observed which is detrimental because it acts as a stress concentration site. The microstructure from the weld zone showed severe intergranular corrosion degradation. Micro cracks and cracks have been observed at several locations, mostly originating from the weld zone. From the EDS analysis of the failed sample it is observed that there is a deposition of Cr along the grain boundary. From the mode of failure it indicates that probable reason for the premature failure is due to sensitization of the component. In this case, the precipitation of chromium carbides may be occurred during welding operation when the heat affected zone (HAZ) experiences a particular temperature range (550–850 °C). From the microstructure it is observed that the welding operation was not proper and there is every chance that there is heat generation in around sensitization range leading to precipitation of chromium carbides consumed the alloying element – chromium from a narrow band along the grain boundary and this makes the zone anodic to the unaffected grains. The chromium depleted zone becomes the preferential path for corrosion attack or crack propagation if under tensile stress. Thus it leads to premature failure of the component during service.