An ASTM A193-83a grade B7 (AISI 4140) steel turbine impeller shaft fractured after 2 months of service. Failure had initiated at three separate points around the periphery of the shaft, each associated with one of three keyways. SEM fractography, metallography, and chemical analysis indicated that the mechanism of fracture initiation was torsional fatigue. Intermittent deceleration and acceleration resulting from power surges during operation of the turbine caused torsional vibration and was considered the most probable source of the required cyclic stress. Final failure took place by torsional shear.