Changes in microhardness, structural-phase state, degree of the lattice microstrains, level of residual compressive stresses and tribological properties of stainless steel 40Х13 (AISI 420) after ultrasonic impact treatment (UIT) of surface layers in air and inert media have been studied. The increase in the surface microhardness of the samples treated in argon and air reaches $\sim$ 2.4 times (HV100 = 5.8 GPa) and $\sim$ 2.8 times (HV100 = 6.9 GPa), respectively. The consequence of UIT in an inert environment is a decrease in the roughness parameters of the modified surface in contrast to the treatment performed in a chemically active environment (air). The latter leads to the formation of a more developed microrelief. The difference in the roughness characteristics is due to the mechanochemical oxidation of the air-UIT processed surface and deformation-induced formation of Fe$_{3}$O$_{4}$ and Cr$_{2}$O$_{3}$ oxides. The developed surface microrelief containing oxides, which increase hardness and can act as a solid lubricant, provides a significant reduction in the friction force as compared to the initial state and the surface processed in an inert environment.