Abstract
Top gas recovery turbine (TRT) is used to utilize high top pressure for power generation in modern blast furnaces. A failure occurred in the TRT system that led to interruption in operation and reduced power generation. A large increase in vibration of turbine discharge-side bearing was reported prior to the failure. Failure investigation consisted of collection and analysis of plant data, on-site and visual observations, chemical analysis, microstructural analysis using optical and scanning electron microscope coupled with energy-dispersive spectroscopy and hardness measurements. Both the failed components, that is the connecting piece and the rotor blade, were composed of AISI 420 martensitic stainless steel. In the failed connecting piece, fatigue fracture characterized by ratchet and beach marks were identified to occur from a step, a potential stress concentration site. Analytical scanning electron microscopy revealed that the fatigue cracks originated from the corrosion pits having iron oxide scale rich in sulfur and chlorine. Pitting occurred due to breakage of passive chromium oxide film due to acidic water in the system containing high concentration of chlorine. Crack propagation occurred predominantly along the chromium carbide/tempered martensitic interface. Stator blades failed subsequently in brittle mode. Lower hardness of the connecting pieces compared to the stator blade would have promoted fatigue failure in the former component. Based on the analysis, primary mode of failure is termed as “corrosion fatigue.” Practical recommendations are provided to improve the reliability of the system.
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