Effect of ultrasonic shot peening on the fatigue strength of stainless cast steel ASTM CA6NM for hydraulic turbine runner

In this paper, in order to investigate the effect of the ultrasonic shot peening (USP) treatment on fatigue characteristics of the structural materials for hydroelectric facilities, plane bending fatigue tests were carried out using stainless cast steel ASTM CA6NM performed by USP treatment. The fatigue test results showed that the fatigue strength of the USP materials was approximately 60% higher than that of the untreated materials. In order to examine the reason for that, the effect of USP treatment on fatigue crack initiation behavior was evaluated based on the Haigh's diagram. This evaluation implied that increasing the crack initiation resistance associated with high hardness and high compressive residual stress at surface by USP treatment is the major cause for the improvement of fatigue strength. Furthermore, the effect of USP treatment on fatigue crack propagation behavior was also examined by simulating the crack growth rate considering the stress intensity factor at crack tip. Crack propagation simulation results suggested that compressive residual stress leads crack growth rate slow compared with untreated material.