Nanocrystalline structure formation in EK-181 steel surface layers on ultrasonic treatment

The microstructure and the phase composition of surface layers of ferritic-martensitic EK-181 steel (Fe-12Cr-2W-V-Ta-B-C) subjected to various combinations of ultrasonic and heat treatment were examined. It is shown that a nanocrystalline a-phase structure with grain boundaries on which vanadium carbide particles precipitate is formed only on intermediate ultrasonic treatment between quenching and ageing. The intermediate ultrasonic treatment provides the greatest increase in the yield limit of the test specimens under uniaxial tension. A correlation is found between the microstructure, the mechanical characteristics and the fracture mode.