Recurrent premature failures of low pressure (LP) steam turbine blades were reported from a 220 MW nuclear power plant. The failure was noticed only on the fifth stage on the low-pressure side. The turbine blades are fastened to the rotor by "fir tree" type roots. The failures were always found to be of multiple origin and initiated at a distance of approximately 15mm from the free edge, on the side flanks of the fir tree root.Detailed failure investigations were conducted using a variety of test techniques. The failure initiation zone was seen to have multiple crack origins associated with beach marks. In addition a number of corrosion pits and broach marks were observed on the side flanks of the fir tree. Metallography revealed presence of stringer and globular type of inclusions in the material, which were confirmed to be MnS and alumina. The general microstructure was tempered martensite with some regions showing untempered martensite near the crack initiation zone. Residual stress measurements, revealed high tensile residual stresses, on service exposed blades from the same stage, indicative of high rate of fatigue loading. The surface residual stress on the fresh blade was compressive in nature.This paper discusses the details of results obtained during the failure analysis and suitable recommendations made to avoid such failures.
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