Cavitation erosion of hydraulic turbine steels containing 0.2 pct carbon and 1 pct Mn

Abstract

A vibratory ultrasonic apparatus has been used to study the cavitation erosion resistance of 0.2 pct C steels used in the construction of large hydraulic turbines. In the “fabricate weld” technology the final normalizing operation improves the brittle fracture resistance, thereby improving the reliability of the turbine over that of the stress relieved machine. However, normalizing improves the cavitation erosion resistance of some steels only, while a slight degradation occurs in the case of others. Materials properties such as fracture toughness, Charpy impact energy, average grain size, and strain energy to fracture were found to have no relationship to the cavitation erosion resistance of the material. In the range of microstructures observed in hydraulic turbines, the cavitation erosion resistance is determined by the ultimate resilience, i.e., a high ultimate tensile stress is associated with a low cavitation erosion rate. The mechanism of material removal consists of plastic deformation and ductile rupture. Twinning and brittle fracture do not play a significant role in the cavitation erosion of these steels.