Failure Analysis of Vertical Turbine Pump Shaft

A prematurely failed vertical turbine pump shaft was investigated for root cause analysis. The analysis involves operating conditions, visual examination, chemical composition, bulk hardness, microstructure, and fractured surface analysis. Visual examination clearly showed that the shaft had failed at the step region; however, no evidence of fatigue striations/beach marks was seen on the fractured surface. The chemical composition matched with the ASTM A276-410 (hardened and tempered), but, higher carbon content was found than the specification. The tempered martensitic microstructure indicates that the steel was heat-treated. The bulk hardness of the shaft was found to be 314-319 HB, which corresponds to 538 degrees C tempering, which is embrittlement zone of the steel, as can be found from the literature. The fractured surface of the shaft clearly showed intergranular brittle fracture, which arises due to embrittlement. Hence, it is inferred that the tempering heat treatment temperature was wrongly chosen (538 degrees C) leading to temper embrittlement, which resulted in premature failure of the shaft in service.