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.