Investigations on fatigue curves in dependency of water quality for nickel-martensitic steels

Abstract

With the need for flexible operation, Francis runners are exposed to various operating conditions outside the traditional operating range. To grant sufficient fatigue life, a deep knowledge of both loads and material behavior is mandatory. With the change in operation diverse stress amplitudes and number of cycles have to be considered for the comparison with fatigue curves. Hydro power turbines are exposed to possible corrosion attack due to the surrounding water environment, which is a relevant impact on the fatigue curves. Therefore an overview of parameters that influence the corrosivity will be given. From an evaluation of a database that contains water quality data of worldwide hydro power projects, different water qualities are defined to perform fatigue tests with the influence of a corrosive medium. For selected water qualities, preliminary material test results will be presented.

Corrosion fatigue investigations on cast, forged and welded nickel-martensitic steels for hydro turbines

“…Within a contribution that has been published in 2021 [1], a general approach and the testing setup are described and preliminary results of an extensive corrosion fatigue testing program are presented. These activities are part of a joint research project of VH and the Materials Testing Institute University of Stuttgart (MPA).…”

“…This procedure was selected in order to cover a vast range of both, load amplitudes and number of load cycles until failure. Fatigue results of a typical cast steel for hydro turbines were presented in [1] comparing both water qualities. Whereas at an R-ratio of 0 a benefit was obvious for the less corrosive water, data situation was not yet sufficient for final evaluation of the R-ratio of 0.5.…”

“…All preliminary results presented in [1] were gained using a nickel-martensitic stainless cast steel with 13 % Chromium and 4 % Nickel (GX4CrNi13-4, 1.4317 acc. to DIN EN 10283 or CA6NM acc.…”

“…The determined stresses at critical spots are then subsequently incorporated in modern fatigue assessment concepts in order to proof sufficient safety against fatigue failure of the components for a given load universe. At Voith Hydro (VH), for example, a sophisticated Operation Cycle Concept for the fatigue life prediction of Francis runners has been established within the last years [1,2,3]. For a reliable fatigue assessment of a hydro turbine runner, both an accurate prediction of loads and a profound material database covering different loading conditions as well as environmental influences (e.g.…”

“…Typically, the material characteristics are implemented into the fatigue assessment procedure in terms of a so-called Haigh-Diagram (allowable stress amplitudes vs. mean stresses) which is derived from corresponding SN-curves with different mean stress levels or different R-ratios. Hereby, the R-ratio is defined by the lower stress (min) and the upper stress (max) or alternatively by the mean stress (m) and the stress amplitude (a), respectively, see equation (1).…”