The purpose of this work was to determine the essence of the influence of the impact velocity (5, 7, and 9 m/s) on Hertzian stresses and the erosion mechanism of ferritic-pearlitic S355J2 steel. The investigations were carried out using a slurry pot tester. S355J2 steel showed a strong sensitivity to changes in impact velocity. A significant increase in erosion rate was observed at a velocity of 9 m/s. This increase was 5-fold and over 15-fold compared to velocities of 7 m/s and 5 m/s, respectively. The study of Hertzian stress is crucial in erosion research because it helps understand how impact energy is absorbed by the eroded material and the mechanisms that cause surface wear. A linear increase in mean contact pressure and maximum shear stress was observed with increasing impact velocity. The mean contact pressure increased from 4.3 GPa to 5.5 GPa and the maximum shear stress increased from 2.0 GPa to 2.5 GPa. The kinetic energy of the solid particles that hits the eroded steel is distributed in the contact area, which leads to various deformations and wear mechanisms. The primary type of deformation was fatigue degradation of the surface layers of the eroded steel. The high kinetic energy of solid particles contributed to the formation of plastic deformations and strongly deformed steel flakes. Higher impact velocities generally result in greater forces and contact stresses on the material surface. This led to the intensification of plastic deformation in the contact areas and increased the Hertzian stresses.