Blade fractures and cracks occurred on low pressure second last stage moving blade of a steam turbine before and after the blade optimization. In order to find out the cause of this type blade failures and prevent subsequent reoccurrence

the blade failure

operating parameters and historical records were checked

and the materials and fractures of some failed blades were analyzed through physical and chemical inspection. Moreover

the centrifugal stress of the blade and the vibration characteristics of the gear train before and after optimization were numerically analyzed by finite element method. The results show that

the blade fracture is a high peripheral fatigue fracture. Before optimization

the main reason for cracks and fractures at the connection transition between the top of the inner cambered surface and the shroud on the steam outlet side of the blade is that the blade has a large torsional recovery under working conditions

resulting in severe compression of the shroud

and stress concentration and fatigue damage occur at the connection transition between the top of the inner cambered surface and the shroud on the steam outlet side. The unreasonable design of blade root structure is the main factor for high cycle fatigue cracking of blade root

while the vibration of the sixth pitch diameter of the first stage of blade impeller system falling into the “3-coincide point” resonance area is the secondary factor for blade failure. After optimization

the main reason for the fracture is the unreasonable design of the blade root structure

and the vibration of the eleventh pitch diameter of the second stage of the blade impeller system falling into the “3-point” resonance area is the secondary factor causing the blade root failure.