Investigation of surface oxidation of low carbon sheet steel during its treatment with Nd:Glass pulsed laser

Abstract

The surfaces of hot-rolled and cold-rolled samples of low carbon mild steel have been treated by Nd:Glass laser. The temperature conditions at the surface have been characterised via a numerical simulation. The thermal conductivity equation has been solved for the one-dimensional case. In the model developed for numerical simulation the change in absorption coefficient during the laser treatment due to the formation of surface oxide film has been considered. The temperature gradients during the laser pulse and following the cooling stage have been estimated. It has been found that the maximum temperature reached at the end of the laser pulse on the samples’ surface is 1814 °C, and this relatively high temperature causes oxidation of both samples and surface melting of the cold-rolled specimen. The thickness of the formed oxide film has been evaluated via the developed model for numerical simulation to be 77 nm. Some important parameters of the grown oxide film namely: phase composition; lattice imperfections; and residual macro-stresses have been estimated via X-ray asymmetric grazing incidence beam methods. It has been revealed that the oxide film has a layered macrostructure built up by two mono-phase layers namely Fe 3O 4 and FeO, from which the former is the top and the latter the bottom layer. The comparison between the broadening of the corresponding diffraction lines shows that the FeO layer formed on the cold-rolled steel has a higher concentration of lattice imperfections than the one grown on the hot-rolled specimen. For the top Fe 3O 4 layers formed on both steels the relation between the peaks broadening is the reverse. It has also been revealed that the grown laser-assisted oxide films contain tensile residual macro-stresses.