Surface hardness of mild steel (AISI/SAE 1020) after direct laser surface alloying with chromium.

Abstract

The process parameters of laser surface alloying of low carbon steel (AISI/ SAE 1020) with chromium powder have been studied. A powder feeder and an off-axis nozzle were used to project chromium powder over the base material while it was simultaneously molten by the laser beam. Geometry of the meltpool, the resolidified microstructures and the resulting hardness profiles were determined by metallographic analysis and phase composition determined by X-ray diffraction (XRD). The obtained layers were identified as Cr-Fe alloy with a hardness increased by a factor of 1.4 to 2.6 with respect to the steel substrate. This result is attributed to the thermal load and cooling rate experienced by the surface and to a solid solution effect due to the high chromium content. None of the two effects have modified the bulk material. The result is relevant for practical uses because this type of steel grade cannot be hardened by conventional methods.Keywords: CO2 laser, laser surface alloying, mild steel, surface melting, microstructure, Fe-Cr alloy, rapid solidification