The last stage low pressure LP steam turbine blade operated for 3000 rpm was failed after 42000 equivalent hours of operation from 600 MW thermo electric plant. The fractured blade was investigated and compared with virgin blade to determine the failure mode. Visual examination, chemical analysis, uni-axial tensile, V-notch impact tests, bulk hardness, EDX, fractography and microstructural characterization were conducted on the fractured blade. Further, wet fluorescent magnetic particle inspection and surface roughness measurement conducted on virgin blade as well. Initial visual analysis suggested that chevron cracking accompanied with several ratchet marks. Moreover, dynamic analysis of last stage virgin blade was performed and evidenced that natural frequency was stable. Modal analysis had predicted using Finite Element Analysis. Both experimental and theoretical frequencies had been closely matched, and natural frequencies were well below the resonant frequency, thus, vibration had not induced fatigue fracture. Moreover, fractured blade was fractographic and metallographic analyzed for fatigue fracture characterization. An engineering failure analysis suggested that several non-metallic inclusions have been de-bonded at crack origin zone. Multiple source of fatigue cracks have been initiated adjacent to material anomalies and fatigue fracture propagated by alternating centrifugal induced tensile stress. Fine curved striations have been evidenced on fatigue crack initiation and propagation zones. The blade exhibited tempered martensite and tensile properties including hardness were within the specifications. The presence of anomalies including non-metallic inclusions and internal volumetric material defects has been linked with poor blade toughness which had reduced the fatigue resistance of last stage blade. Interaction of manganese sulfide inclusions with complex alternating centrifugal and bending stress had induced fatigue fracture. Several recommendations including blade manufacturing by clean steel technology are suggested based on various obtained evidences to prevent LPT blade failures in power plants.
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