4. Conclusions and recommendations

The metallographic structure composed of low carbon martensite + dispersed intermetallic strengthening phase is normal. It can be seen that the crack originated from the corrosion pit on the inner arc surface .
The fracture of the turbine blade is a typical corrosion fatigue fracture. The local point corrosion caused by steam containing Cl-, S2 -, K + etc. occurs on the blade, forming a corrosion pit as the corrosion fatigue source. At the same time, the complex periodic alternating stress is formed due to the tension, bending, torsion, exciting force of the blade, resulting in high cycle fatigue(HCF), and finally brought about the corrosion fatigue fracture of the turbine blade.
The formation of corrosion pits is an electrochemical process with multiple ions and is affected by complex alternating stress. On the one hand, the stress affects the corrosion process and the formation of corrosion pits by affecting the anodic dissolution. The width and depth of corrosion pits directly affect the critical stress field intensity factor and critical strength of crack. On the other hand, the complex alternating stress directly promotes the crack growth until the fracture failure after the formation of corrosion pit.
It is suggested to strengthen the control and monitoring of water quality to reduce the corrosive ions in the water; it is necessary to avoid the emergency start, emergency stop and large changes of working conditions, minimize the alternating stress and unit vibration, and finally reduce the occurrence of high cycle and low cycle fatigue during the operation process.
It is suggested that application of suitable coating or surface modification treatment for anti-corrosion and anti-fretting on blades root surface is carried out,such as the shot peening or thermal spray metal-matrix composite coatings.