989 By studying the deformation of a surface layer, we may judge its interaction in a joint, predict its effectiveness, and recommend methods of hardening this layer so as to improve wear resistance. In the present work, we study the formation of the structure and phase composition in ultrasonically modified surface layers of Γ 13 Hadfield steel, in dry friction [1, 2]. The samples are tested on a 2168UMT1 unit in conditions corresponding to shaft–bush, end-seal, and disk–cam junctions. The test parameters are as follows: pressure up to 4 MPa; slip rate up to 0.3 m/s; test time up to 6 h. Ultrasonic hardening is by means of a 20-W 160UZTK-18/22-0/25 unit (frequency 22 kHz, amplitude 15 μ m) [3]. This unit ensures plastic deformation at depths up to 700 μ m. The samples are investigated by X-ray structural and X-ray phase analysis and by optical microscopy. In the initial state, the Hadfield steel has an austenitic structure and consists of a single phase (Fig. 1). The X-ray diagrams of the samples after tribological tests at 3 MPa are characterized not only by broadening of the γ phase but also by the appearance of reflections close to 2 θ = 40 ° , which correspond to cementite Fe–Mn–C (Fig. 1, curve 2 ). The appearance of new phases may evidently be due to considerable heat liberation at the surface. The appearance of a faint reflex close to 2 θ = 77 ° is due to α -phase formation in friction. Whereas the hardness in the initial state is 3200 MPa, it is 8400 ± 200 MPa at the frictional surface. This cannot be associated with the formation of deformational martensite but is determined by changes in the fine structure, as noted in [4].