ステンレス鋳鋼水車ランナSCS5の疲労強度に及ぼすキャビテーション壊食による表面損傷の影響評価

概要

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In hydraulic turbine runners, accidents due to cyclic loading often occur. In this study, tension-compression and bending fatigue tests were carried out for the stainless cast steel SCS5 cut out from the hydraulic turbine runner vane used for 22 years, in order to clarify fatigue properties and fracture mechanism of SCS5 with cavitation eroded surface. The test results are summarized as follows : (1)The S-N curve of aged SCS5 used in this study has wide scatter and fatigue strength of the specimen with cavitation eroded surface was lower than the specimen data cut out from vane inside. (2)From SEM observation of the fracture surfaces, crack initiation sites were found to be the cavitation eroded surface and the casting defect which consist of dendritic phase and pore. (3)It is possible to evaluate the fatigue life of aged SCS5 by using the defect size √<span style="text-decoration: overline;">area</span> and the initial stress intensity factor calculated by cavitation damage size and casting defect size. The value of the threshold stress intensity factor of cavitation eroded surface obtained in this study were 3.6 MPa·m1/2. (4)It was proposed that stress intensity factor of cavitation predicted using maximum height of surface roughness, because damage size √<span style="text-decoration: overline;">area</span> of cavitation erosion was proportional to defect depth.