Failure Analysis Case History No. 014

The blades from aluminum bronze pump impellers were failing after a short time in service due to complete separaton fracture. The blade failures resulted in disruption to the plant operation and expensive repairs.

Dye-penetrant inspection of the impeller revealed many areas of random, PT indications over much of the impeller blade and hub. Visual examination of blade fractures revealed thumbnail areas where the fracture was a different color from that of the overload fracture. Fractures initiated from the "loaded" side of the impeller blade. SEM examination of a fracture revealed fatigue striations at the initiation edge of the blades. Microstructural examination of a blade at a fracture revealed that dealuminification had occurred in service,. reducing the strength of the blade material in the dealuminified areas. The impeller blades had suffered fatigue at weak, dealuminified areas near the blade hub. The dealuminification was due to exposure to the filtered water environment.