Comparing the surface hardness of mild steel processed with CO₂ and fibre lasers

The microhardness of low carbon mild steel was analysed after surface treatment by using carbon dioxide CO₂ and Fibre laser for comparison. Identical input parameters were used to compare the increase between the as-supplied material hardness and the treated surface. Microhardness testing was used to measure the surface hardness and a scanning electron microscope (SEM) was used to investigate the microstructural changes in the substrate. It was found that the laser scanning speed had a significant effect on the increased surface hardness, while the laser power was a less significant factor. The samples processed with the lower laser fluence of the Fibre laser exhibited a hardness increase of 38% with maximum depth of 200 μm, while those processed by the CO₂ laser exhibited a hardness increase of 41% with a maximum depth of 50 μm. A full design of experiment (DoE) model was modified for the optimization and prediction of the laser hardening process. The processing parameters applied were the laser beam power and the scanning speed.