Evaluation of rendering fatigue crack harmless using needle peening for hydraulic turbine runner of stainless cast steel SCS6

Abstract

In hydraulic turbine runners, accidents due to cyclic loading often occur, which are caused by large defects and residual stress introduced while making welding repairs of a part eroded by cavitation. In this study, we have aimed at improving the fatigue property of SCS6 stainless cast steel which has a propensity to develop large cracks and other defects, and bending fatigue tests were carried out to characterize the fatigue properties and fracture mechanism of the SCS6 when subjected to needle peening. The test result are summarized as follows: (1)The bending fatigue limit of 107 cycles on materials that underwent needle peening improved by 60% compared to that of untreated materials. A semi-circular slit measuring 2c=3.0mm and a=1.5mm is rendered harmless by needle peening. The high stress amplitude below the bending fatigue limit of 107 cycles, even if repeated, does not relax the residual compressive stress. (2) The bending fatigue limit is not affected by fatigue strength in the peening direction. Fatigue strength does not decrease even with a small amount of peening at a right angle and in a parallel direction. (3) It is possible to evaluate the fatigue life of the SCS6 by using the defect size √area and the initial stress intensity factor range calculated from the sizes of the defect and of the artificially induced semi-circular slit. The value of the threshold stress intensity factor range ΔKi･th for the defect corresponds to ΔKi+R･th, its value for the artificially induced semi-circular slit created by needle peening.

Keywords

• surface improvement
• stress intensity factor
• residual stress
• fatigue
• power generation