Fracture analysis of a cooling water pump shaft

Six shaft failures were encountered in a centrifugal water pump, part of the engine cooling system of a container ship. The last two failed pieces were received for analysis. A detailed study was carried out to determine the cause of the component failures. Historical data was collected, visual inspection was performed and a photographic file was created. The pieces were measured in order to record their general features and a 3-D model was generated. Hardness measurements were carried out. Microstructure was examined by means of light microscopy. A finite element simulation was conducted in order to determine the stress topology and to identify possible critical areas. Chemical analysis was carried out. The shafts were manufactured onboard, from AISI 304 stainless steel. The low hardness of both pieces indicates insufficient mechanical properties. Microstructural examination showed characteristic microstructure of coarse austenite. The presence annealing process derived twins and limited grain-boundary carbide precipitates were verified. The shafts failed due to torsional fatigue. Fracture initiated at the keyway on the propeller side as it was predicted from the finite elements analysis. Material selection material has and poor machining were found to be the main cause of failure. Appropriate recommendations were provided.