Mechanisms of regulating hardness distribution and tribological behavior through laser polishing

In this study, a pulsed laser was used to polish the surface of mold steel to investigate the effects of laser polishing on hardness and tribological behavior. The surface roughness of laser-polished samples was reduced to varying degrees under different single-pulse energy densities (E-S). After laser polishing, the hardness of the outermost layer of the samples decreased, while along the depth direction, the hardness first increased, then decreased, and finally stabilized to match that of the base material, around 724.89 HV. The thickness of the surface softening zone increased with higher energy density, with softening mainly occurring in the remelted layer. When E-S > 3.5 J/cm(2), the minimum hardness in the softening zone was 324 HV, primarily due to >70 % complete austenitization of the outermost layer. At energy densities of 2.5 J/cm(2) to 3.0 J/cm(2), softening was mainly associated with approximately 10 % austenitization in the lower part of the remelted layer. Additionally, the hardening of the heat affected zone (HAZ) was influenced by increased geometrically necessary dislocations (GND) density and grain refinement. Further analysis of the coefficient of friction (COF) curves revealed that the COF of the laser-polished samples showed less fluctuation over time than the untreated samples by 600 s. Unlike the abrasive wear mechanism observed in untreated samples, the wear mode of laser-polished samples predominantly involved a combination of abrasive and adhesive wear. Except for E-S = 2.5 J/cm(2), the wear rate of laser-polished samples increased. At an energy density of 2.5 J/cm(2), the surface roughness Sa decreased from 2.40 mu m to 0.54 mu m, and wear reached the hardened layer, resulting in a reduced wear rate. This work provides new insights into controlling the wear resistance and hardness of laser-polished mold steel through microstructural control.