In the present work, the slurry erosion behavior of friction stir processed (FSPed) hydroturbine steel (CA6NM) was investigated. For comparison, the erosion performance of unprocessed CA6NM steel was evaluated under similar conditions. Friction stir processing (FSP) is a microstructural refinement tool which is useful in enhancing the bulk and surface properties of materials. An in-depth characterization of both steels was done using an optical microscope (OM), a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), the electron backscatter diffraction (EBSD) technique, and micro- and nano-indentation techniques. The FSP of the steel helped in reducing the erosion rates by 50% to 60%, depending upon the impingement angle. The improved performance of the FSPed steel in comparison to unprocessed steel was attributed to microstructural refinement, which increased the hardness and yield strength. At an oblique impingement angle, plowing, along with microcutting, was observed to be the dominant erosion mechanism. At a normal impingement angle, the material removal process was controlled by the platelet mechanism of erosion. A modified form of the mathematical model for predicting the erosion rates of the ductile materials, proposed by authors earlier, was also presented. This modified model based upon the theory of plasticity was able to predict the erosion rates with an accuracy of ±20%.