Analysis on corrosion fatigue cracking mechanism of 17-4PH blade of low pressure rotor of steam turbine

The causes of many blade fractures of a steam turbine in a power plant were studied by means of macro analysis, mechanical tests, metallographic examination, SEM and X-ray fluorescence spectrum analysis (XRF). The results show that the blade cracks due to corrosion fatigue. Cl-,K (+)react with the turbine blades in the steam environment in physical, chemical and electro-chemical ways, causing local spot corrosion on the blades, forming corrosion pits. In addition, the steam condensated 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+ react with the turbine blades. Under the action of combined 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 blade cracks are determined by threshold nominal stress range for crack elongation Delta sigma th, which depends on the depth and width of corrosion pit. The combined alternating stress directly promotes the crack propagation until the fracture fail under the external conditions of alternating stress formed by tensile force, bending force, torsion force, and exciting force.