Microstructure Evolution and Recrystallization of D36 Steel during Ultrasonic Impact Assisted with Electropulsing and Heat

Ultrasonic impact treatment (UIT) combined with high-energy electropulsing (EP) was applied to D36 low-carbon steel with three different electrical regimes. Submicron crystalline was obtained on the superficial region after the treatment due to continuous dynamic recrystallization. The cementite experienced strain-induced decomposition and precipitation. The microstructure is significantly determined by the current density and temperature. A strengthened layer with a maximum hardness of 285 HV was obtained in EP-UIT, in comparison with the hardness of 227 HV resulted from UIT solely. Alongside with high hardness, the strengthened layer extended to a remarkable depth of nearly 2 mm due to acoustic softening, electroplasticity and thermal softening engaged simultaneously. A 3-μm oxide layer in average consisting of magnetite and hematite formed on the treated surface. Joule heat and athermal effect of EP are the factors inducing these phenomena.