LASER IGNITION OF BULK IRON, MILD STEEL, AND STAINLESS STEEL IN OXYGEN ATMOSPHERES

The ignition of pure iron, mild steel S355J, and stainless steel 316L has been investigated. The whole ignition and combustion processes have been monitored using a high-speed video camera and adapted pyrometry. Our results show that the absorptivity of the iron and mild steel to laser radiation increases rapidly at 850 K, from 0.45 to 0.7, and that of stainless steel increases more gradually during the heating process from 0.45 to 0.7. The ignition of iron, mild steel, and stainless steel is controlled by a transition temperature, at which the diffusivity of the metal increases sharply. The transition temperature of pure iron and mild steel is around 1750 K, when molten material appears, and that of stainless steel is around 1900 K, when the solid oxide layer loses its protective properties. These temperatures are independent of the oxygen pressure (from 2 to 20 bar) and of the laser intensity (from 1.6 to 34 kW.cm(-2)). During ignition, the temperature increases very strongly at first, and after that a change in the heating rate of the surface is observed. A diffusive-reactive model, provided with equations describing the diffusion of oxygen in the metal and the transfer of heat released by the oxidation reactions has been solved. The model correctly reproduces the sharp rise of temperature as well as the decrease in the heating rate that follows. Comparison between calculated and experimental data shows that, without liquid convection flow in the melt, combustion would extinguish as soon as the metal surface is fully oxidized and that the combustion front moves into the metal.