Dynamic recrystallization pathway transition and subsurface microstructure evolution during ultrasonic vibration-assisted gear grinding of 12Cr2Ni4A steel

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Controlling the subsurface microstructure by introducing ultrasonic vibration during the machining process is essential for achieving the surface integrity required for fatigue resistance. This study investigates how vibration-induced intermittent contact and dislocation evolution jointly affect the competition between continuous and discontinuous dynamic recrystallization (CDRX/DDRX) in 12Cr2Ni4A gear steel during ultrasonic vibration-assisted gear grinding. A competitive evolution model was developed by coupling two mechanisms to capture these ultrasonic effects: (1) the thermomechanical history driven by gear geometry and intermittent grinding motion; and (2) acoustic softening-modified dislocation dynamics governing substructure formation. Model predictions of thermal response and grain refinement were validated through temperature measurements, EBSD, and TEM. Results indicate that ultrasonic vibration reduces thermomechanical loads while producing a finer subsurface gradient microstructure. This is primarily attributed to intermittent contact and enhanced dislocation motion shifting the process toward CDRX-dominated recrystallization, thereby promoting the formation of dislocation cells and subgrain rotation. The influence of machining parameters on grain structure characteristics was further analyzed. This work provides a practical framework for tailoring refined surface layers in the grinding of high strength gear steels.