Effect of ultrasonic-assisted surface plastic deformation on the microstructures and tensile properties of discrete laser hardening treated Cr-Ni-Mo alloyed steel

Discrete laser surface hardening (DLSH) can form an array of separate high hardness zones with excellent wear resistance on the surface of ferroalloys. However, the soft self-tempering zone at the subsurface layer with slow thermal diffusion seems to be unavoidable and harmful to the workpiece. In this study, the microstructure evolution and tensile properties of martensitic Cr-Ni-Mo alloyed steel after discrete laser surface hardening (DLSH) and further combined ultrasonic-assisted surface plastic deformation (USPD) treatment were investigated by the X-ray diffraction (XRD), electron back-scattered diffraction (EBSD), transmission electron microscope (TEM) and electronic universal testing machine. Results show that the combination of USPD treatment changed the phase, dislocation density, amount of twins, and texture component in the DLSH treated surface layer and caused a difference in the strengthening mechanism between laser hardened zone (HZ) and substrate zone (SZ). Because tensile cracks tend to initiate first in the subsurface soft zone outside HZ, the tensile strength and yield strength decreased by 10.4% and 4.7% after DLSH treatment. After USPD treatment, the surface gradient plastic deformation improved the subsurface strength of soft zone, and the threshold value of crack initiation increased accordingly. Finally, the effective inhibition of crack initiation at the surface layer caused a stable back stress hardening effect, and significantly increased the yield strength and tensile strength of the material by 5.1% and 3.7%, respectively, compared with the untreated sample. The DLSH and USPD treatment decreased the continuous deformation ability of the sample surface, and the fracture surface showed a brittle fracture.