Measurements and prediction of extreme defect distributions for fatigue assessment in multi-pass weld of 13%Cr–4%Ni alloy for hydraulic turbines

Flux cored arc welding (FCAW) is a manufacturing process commonly used for hydro-turbine application, especially in the assembly of Francis runners made of 13%Cr–4%Ni stainless steel. The welded connection between the blade and band/crown is particularly susceptible to high stress levels due to the design of the runner and the presence of welding defects which act as localized stress concentrations. This research aims to predict the effect of these defects on the performance of the runner during operating condition. To investigate the occurrence of the defects, the distributions of the observed discontinuity are measured and analyzed on T-shape joint specimens using metallographic and X-ray 2D images. The data obtained from these two methods of acquisition are compared to identify any discrepancy. Extreme Value Statistic (EVS) is used to estimate the likely occurrence of the largest defects within the welded runner zone. Based on these findings, a probabilistic Kitagawa–Takahashi diagram with the El-Haddad approximation is adopted to quantify the influence of the defects size in the fatigue assessment.