Analysis on corrosion fatigue cracking mechanism of 17-4PH blade of low
pressure rotor of steam turbine
Abstract
Abstract: The causes of many blade fractures of a steam turbine in a
power plant were analyzed by means of macro analysis, mechanical
examination, metallographic examination, SEM and X-ray fluorescence
spectrum analysis(XRF). The results show that the blade cracks due to
corrosion fatigue. Cl -, K +, etc. react with the turbine blades in the
steam environment in physical, chemical and electrochemical ways,
causing local spot corrosion on the blades, forming corrosion pits.In
addition, the steam condensate has an erosion effect on the blades, both
of which form a corrosion fatigue source. The autocatalytic process of
block cell is formed when Cl -, K +, etc. react with the turbine blades.
Under the action of complex load, the current density i of activation
dissolution of metal can be expressed as a function of complex stress
state .Crack growth has an important relationship with stress, depth and
width of corrosion pits. The larger the width of corrosion pit is, the
deeper the depth is, the smaller threshold nominal stress range for
crack elongation 〖∆σ〗_th is, the easier the blade cracks. Under the
external conditions of complex alternating stress formed by tensile
force, bending force, torsion force and exciting force, the complex
alternating stress directly promotes the crack propagation until the
fracture failure. Keywords:XRF;Corrosion fatigue;Corrosion pit;Threshold
nominal stress range for crack elongation 〖∆σ〗_th;Complex alternating
stress