Application of Casting Simulation in Failure Analysis of Impeller

Commercial programs using finite element analysis (FEA) have been developed for casting design process. However, their applications have been limited mostly during the casting design stage. This paper explores the possibility of using casting design software for failure analysis. An impeller was failed during services by impact load. The fracture occurred at the thin shroud area of the impeller and showed brittle appearance. Hardness test shows differences of hardness number between thin area (shrouds) and thick area at the impeller, resulted from microstructure differences at the impeller. Shrouds area contained high percentage of graphite (type B) and several undercooled phases (type D graphite) which is typical result for high cooling rate solidification. This microstructure reduced the mechanical properties. SEM examination shows transgranular fracture which is typical for brittle fracture mechanism and reveals higher graphite content at shrouds. Accumulation of titanium, calcium and silicon at the center of graphite was detected using EDAX. FEA simulation using Z-CAST confirms the development of microstructure differences which are substantially affected by casting designs and solidification characteristics.