Crack evaluation of the fourth stage blade in a low-pressure steam turbine

A crack of the fourth stage blade in a low-pressure turbine of a 500 MW steam turbine is studied. From non-destructive inspection, the crack was found at the leading edge vane of the fourth stage blade. Material composition analysis, hardness measurement and microstructure analysis were performed to study the cause and process of the cracked blade. On further examination using the replication of the cracked surface for the blade vane, the crack was induced by corrosion pits. It is assumed that the causes of the corrosion pits are the induction of seawater from the condenser tube that has small leakage, and improper water treatment. It is shown that the corrosion pits acted as stress concentration site and facilitated crack initiation under cyclic loading experienced during normal operation of the blade. From the fracture surface of the blade, it is concluded that the crack is initiated from the corrosion pits at the leading edge of the blade vane, and propagated inside by fatigue due to the vibration of the blade.