Effect of Casting Defects on Fatigue Strength of Stainless Cast Steel SCS6 for Hydraulic Turbine Runner

Abstract

In hydraulic turbine runner, accidents due to fatigue often occur. In this study, tension-compression and bending fatigue tests were carried out for the stainless cast steel cut out from the turbine runner crown, in order to clarify fatigue properties and mechanism of fractures on stainless cast steel SCS6 used as hydraulic turbine runner aged 27 years. The test results are summarized as follows:(1)The S-N curve of aged SCS6 used in this study has wide scatter and the tension-compression fatigue strength was lower than bending test data. (2)From SEM observation of the fracture surfaces, crack initiation sites were found to be the casting defect which consist of dendritic phase and pore. (3)It is possible to evaluate the fatigue life of aged SCS6 by using the casting defect size √area and the initial stress intensity factor calculated by defect size. The value of the threshold stress intensity factor obtained in this study were 2.6 MPa·m^1/2. (4)It was proposed that a critical volume of tension-compression test specimen where uniform stress was applied should be 20% of the region from the surface, because stress intensity factor of surface crack is larger than that of interior crack of the same size.