Root cause analysis of bucket stage III OF GT-2A

Abstract

In the present study, examination of fractured gas turbine blades (stage III of GT-2A) with identification numbers 23, 39, 40, 49, 453 and 485 was carried out by different characterization methods. Extensive SEM fractography studies on all the fractured surfaces of ruptured blades indicated that surface crack initiation and propagation at the leading edge was by fatigue mechanism as evidenced by striations, rubbing marks and secondary cracks (which are characteristic features of normal fatigue) whereas the crack propagated thereafter through inter-dendritic/inter-granular path as evidenced by large amount of interdendritic/intergranular facets. Since no corrosion products were found on the pressure side/suction side surfaces, it was believed that hot corrosion was not responsible for the present failure. Further, no features like impact with a foreign object were found thus ruling out the possibility of FOD (foreign object damage). Microstructural characterization revealed that there were some significant changes/damages in the microstructure of these blades after service exposure (but these changes alone cannot result in creep failure). Almost all blades, exhibited gamma prime dissolution band at both concave and convex surfaces, in addition to coarsening of gamma/gamma prime, formation of chain of precipitates, coarsening of precipitates. Though the hardness tests indicated considerable hardening effect due to ageing of these blades at high temperatures over a period of time (further ruling out the creep phenomena as the cause of failure), the stress rupture tests did not reveal any significant reduction in the stress rupture life. Based on the results obtained in the present investigation, it can be concluded that the blades were subjected to surface overheating (above the gamma prime dissolution temperature) for a very short period of time. Because of this, the surface edges lost the gamma prime (i.e. dissolved into the gamma matrix). This is a serious problem because all the mechanical properties would be impaired if the precipitates of gamma prime are dissolved into the matrix at surface. Due to this, fatigue cracks were initiated at the leading edge surfaces and further propagated by inter-dendritic/inter-granular (IG) fracture mechanism. No creep/hot corrosion/FOD was noticed. Therefore the primary cause of failure was surface overheating for a very short period of time and the secondary cause of failure is normal fatigue followed by IG cracking.