Micromelting and its effects on surface topography and properties in laser polishing of stainless steel

This article presents the results of an investigation into using laser for polishing 304 stainless steel (SS) surfaces. The effects on surface topography, reflectivity, hardness, and corrosion resistance of 304 stainless steel were analyzed. An in-house developed finite difference heat conduction model was used to simulate the rapid melting and solidification process. The nonlinear problem of the solid/liquid moving boundary was solved by a novel hybrid numerical method. Melting depth was in the order of submicrons and polishing rate in a range of 5–15 cm2/min. The improvement of surface reflectance could be seen as the result of surface smoothing with roughness reduction. A decrease of surface roughness from 195 to 75 nm was measured using an atomic force microscope. Laser polishing increased the specular surface reflectance to 14%, while diffusive reflectance was reduced up to 70%. The heterogeneous microhardness distribution was transformed into a homogeneous one. Laser polishing could improve the pitting corrosion resistance of 304 SS due to microstructural changes caused by laser rapid melting and solidification. The melting effect was found also useful for sealing of micropores and -cracks and removing of surface scratches.