Metallurgical fatigue failure analysis of the brazed steam turbine blade

In this paper, failure mechanism of a 17th stage blade of an 82.5 MW steam turbine that caused damage to the internal turbine compartment and the adjacent blade equipment has been studied. In order to determine the cause of failure and prevent similar events, various metallurgical and mechanical investigations including chemical composition analysis, metallography and microstructural analysis, fractography using scanning electron microscope and hardness and tensile tests were carried out. The initial results showed that the alloy had a chemical composition, microstructure and mechanical properties in the acceptable range, and the fracture failure was not due to the mechanical and metallurgical degradation during the service. The results of the fractography indicate high cycle fatigue as the main mechanism of the failure and shows that the fatigue crack has initiated from the adjacent hole relative to the vibration damped wire near to the brazing region on the blade, due to inadequate quality and incomplete connection of the brazing and its stress concentration effect the hole, which eventually has propagated and reached a critical level, and a sudden failure of the blade has occurred.