Drilling Performance and Surface Integrity of Hardened 42CrMo Steel in Clean Ultrasonic Vibration Hybrid Drilling Process

42CrMo steel has high strength, good toughness, and other excellent mechanical properties, making it the preferred material for gear, drive shaft, and other key components. However, after heat treatment of such materials, there will still be serious machining problems during the drilling process. Herein, the clean ultrasonic vibration hybrid drilling process, which combines ultrasonic vibration technology with clean cutting technology (using media such as dry, liquid nitrogen (LN₂), cold air, and minimum quantity lubrication (MQL)), is used to investigate the drilling performance and surface integrity of 42CrMo steel. The results show that both cutting force and cutting temperature are reduced under low temperature and MQL conditions compared to dry conditions. At the same time, a notable extension of tool life is obtained under these drilling conditions. MQL demonstrates effective cooling and lubrication properties, enhancing the chip-breaking ability of ultrasonic vibration. In comparison to ultrasonic-assisted drilling (UAD) (dry) conditions, the surface roughness under UAD (MQL) conditions is decreased by 45%, while the maximum microhardness is increased by 13%. The drilling accuracy is significantly improved. Consequently, UAD (MQL) can remarkably improve the hard drilling performance and surface integrity of 42CrMo steel.