Correlation between mass loss on the cavitation erosion and the fatigue stress level for a martensitic stainless steel

Abstract

The effect of fatigue on the cavitation erosion was examined for a martensitic stainless steel used in the manufacture of the blades for the bulb turbines of the “Iron Gates II” Power Plant turbines. For the cavitation tests, two kinds of specimens were manufactured: from the original material (which was not subjected to fatigue) and samples of materials that were subjected to fatigue cycles with different amplitudes (from 170 to 255 MPa). It was observed that fatigue increases the tendency of cavitation erosion and thus, seems like a cumulative damage. The analysis of the cavitation erosion was done considering simultaneously the mass loss of the time dependence, the surface roughness and the modifications to the microstructure of the specimens. For the material previously subjected to fatigue cycles, the final mass loss (after 150 minutes of cavitation exposure) increases approximately by 30% and the depth of penetration by 25%. In the microstructural analyses, numerous cracks at grain boundaries were observed. Finally, a mathematical model for the dependence of mass losses with the applied stress was developed.

Keywords

• cavitation erosion
• cumulative damage
• fatigue
• stainless steel
• water turbine