Popova T. The influence of ultrasonic impact treatment and electric spark alloying on the physical and mechanical properties and structural state of 20GL and 15KSND steel

The thesis is devoted to the study of the regularities of the change in the phase and chemical composition, structure, mechanical characteristics and corrosion properties of low-alloy structural steels under combined action of electric discharge surface alloying and ultrasonic impact treatment. On the basis of X-ray structural and electron-microscopic studies, the regularities of phase and structural transformations in the surface layers of samples were established for a combined processing (EDSA with subsequent UIT) of the surface of low-alloy structural steel 20GL alloyed with nickel, chromium and molybdenum. It is shown that when choosing the alloying elements, one should take into account their solubility in iron, their tendency to the carbides’ formation and their chemical potential, which allows purposefully influencing on the physics-mechanical and anticorrosive properties of the material surface. The physics-mechanical properties of coatings on 20GL steel obtained by the EDSA method were investigated, the methodology for improving the operational properties due to the formation of the necessary structure, microrelief and microhardness after UIT has been developed, which allows choosing the optimum regimes of the combined treatment and provides the highest values of the corrosion resistance and fatigue strength. Hardening of the surface layers of materials is associated with simultaneous reduction of the surface roughness and substantial refinement of microstructure elements, which ensure reduction of the number of the most dangerous technological stress-risers and the growth of the number of low-power structural stress-risers in the surface layers, which leads to the increase in fatigue strength and corrosion resistance. The performed experimental researches on hardening of surfaces of the cast parts of a truck of a freight car indicate to the expediency of using ultrasonic impact treatment to increase their fatigue strength and durability.