Influence of laser melting treatment on surface composition of Fe–17Cr–13Ni single crystal alloy Part 1 Oxidation during laser treatment

The fusion of single crystals of Fe–17Cr–13Ni and Fe–17Cr–14·5Ni–2·3Mo (wt-%) using a continuous wave CO₂ laser does not induce significant crystallographic reorientation; the orientation remains predominantly (100) with misorientations of 3 or 4°. During the laser treatment oxidation can take place, even under helium gas protection. The composition and the in-depth distribution of the oxidised elements (iron oxides and chromium oxide) throughout the oxide layer are studied for the melted zone and for the non-treated surface, using electron spectroscopy for chemical analysis (ESCA). The two main modifications under discussion are the formation of an outer chromium oxide and the surface enrichment of sulphur and molybdenum during oxidation. The oxide thickness and composition are dependent on the heat dissipation rate during the laser treatment. When the energy dissipation is slow the oxide layer becomes thicker (up to 35 nm) and Cr₂O₃ is observed in the outer region of the layer on the heat affected zone and on the non-treated surface. This is not observed on the melted surface area, which is probably because of the vaporisation of chromium. When the alloy contains molybdenum, Cr₂O₃ is also found in the outer region of the oxide layer, even on the melted surface.

MST/1174