The Effects of Laser-Assisted Ultrasonic Nanocrystal Surface Modification on the Microstructure and Mechanical Properties of 300M Steel

Herein, an innovative surface severe plastic deformation method, laser-assisted ultrasonic nanocrystal surface modification (LA-UNSM), is used to treat 300M ultra-high-strength steel. The results show that the synergistic effect of laser heating and ultrasonic striking results in a thicker plastic deformation layer in 300M steel compared with UNSM treatment alone. Moreover, the depth of the deformation layer gradually increases as the laser power level is increased. At the laser power of 68 W, the greatest thickness in the plastic deformation layer (≈53.6 μm) is observed, which is 1.7 times the thickness in the UNSM-treated samples. In addition, the surface of 300M steel forms dense oxide following LA-UNSM treatment and the surface oxide concentration increases with laser power level. Because of the martensitic lath refinement, work hardening, and oxidation, the highest surface hardness is obtained after LA-UNSM treatment at 45 W. As higher plastic strain is induced when a smaller UNSM tip (1.5 mm) is used, the largest and deepest surface plastic deformation layer is obtained in the LA-UNSM-45 W-1.5 mm sample. However, the most significant hardness difference between LA-UNSM and UNSM is observed when the 4.0 mm tip is used. This study shows that LA-UNSM can effectively improve the hardness of high-strength metals.