Quantitative evaluation of 3D surface roughness parameters during cavitation exposure of 16Cr–5Ni hydro turbine steel

Abstract

The cavitation erosion resistance of 16Cr–5Ni grade martensitic stainless steel was evaluated for long periods of up to 35h in a vibratory cavitation test rig as per the guidelines of the ASTM G32 standard. The change in retained austenite content during the initial period of cavitation was monitored by x-ray diffractometry. The evolution of surface topography features and quantitative 3D surface texture parameters were analyzed after different cavitation exposures. The average surface roughness deviations (Sa), standard deviation roughness (Sq), mean roughness depth (Sz) and surface skewness (Ssk) with cavitation time and the corresponding 2D line roughness parameters Ra, Rq, Rz, and Rsk were evaluated using a confocal laser scanning microscope to identify the damage mechanisms in the steel. Also, the rate of change of the surface area and the cavitated volume during cavitation were studied. Three stages of cavitation erosion, such as incubation, acceleration and steady erosion rate based on metal loss rate were determined. A correlation was observed between the change in roughness profiles during the three stages and their respective rates of material loss. The use of 3D surface parameters is an important tool for monitoring progress of cavitation damage in large-sized components.