Abstract
In hydraulic turbine runner, accidents due to fatigue often occurs. In this study, in order to clarify fatigue properties and mechanism of fractures on cast steel SC49 used as hydraulic turbine runner aged 45 years, tension-compression fatigue tests were carried out for the cast steel cut out from the turbine runner surface layer and inside. The results are summarized as follows: (1) The S-N curve of aged SC49 has large scatter although it shows the tendency of the right going down. And also it is found that the fatigue strength of inside part has larger scatter than that of surface layer part. (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 SC49 by using the casting defect size (area)^<1/2> and the initial stress intensity factor calculated by defect size. The value of the threshold stress intensity factor obtained in this study were 4.8MPa・m^<1/2>. (4) The extreme statistics distribution of casting defect size of surface layer was different from that of inside part. The estimate method of the largest defect size in the structural element using the former distribution was suggested.
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