Fatigue failure of low pressure steam turbine blades of a power plant

Recurrent premature failures of low pressure (LP) steam turbine blades were reported from a 220 MW nuclear power plant. The failure was noticed only o n 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 b e 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 tech niques. The failure initiation zone was seen to have multiple crack origins associated with beach marks. In addition a number of corrosion pits and br oach marks were observed on the side flanks of the fir tree. Metallography revealed presence of stringer and globular type of inclusions in the materi al. which were confirmed to be MnS and alumina. The general microstructure was tempered martensite with some regions showing untempered martensite nea r the crack initiation zone. Residual stress measurements, revealed high te nsile residual stresses, on service exposed blades from the same stage, ind icative 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 ana lysis and suitable recommendations made to avoid such failures.