The first time study stainless subjected to prolonged high-frequency (50 MHz) external hydrodynamic effects (EHDE) under high pressure (2-3 GPa) in the device of special design were performed by transmission electron microscopy. It is found that due to EHDE solid surface layers of steel of thickness up to 100 µm undergo a martensitic deformation-induced transformation with appearance of thin twinning crystals a and e phases structure. In following intermediate layers located at a depth of 100-200 µm, traces of fragmentation with dislocation and twinning substructure inside the austenite grains, because of work hardening of austenite in the process of direct and reverse martensitic transition g-e-a were discovered. Features of the g-austenite, e and a-martensite microstructure were studied. Based on the analysis of the obtained and known data it is concluded that due to the multi-cycle high-frequency external impact take a place thermo-, baro - and elasto-plastic mechanisms of martensitic transformations. The martensitic transformation should be accompanied by baro - and magnetocaloric exothermal effects by direct martensitic transformation and, consequently, endothermal effects during reverse transformation.