Laser-treatment-induced morphology and structure modifications of stainless steel: Element segregations and phase evolutions

Abstract The effect of laser treatments on the surface of stainless steel (SS420) has been studied by employing topographic mapping, X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman backscattering spectroscopy, addressing the morphological and structural modifications. The treatments were processed in air atmosphere with varied laser-scanning speed from VL = 40 to 5 mm/s while keeping the laser power constant at 400 W. The topographic evolutions reveal an onset of surface melting when VL is reduced approaching 20 mm/s while the structural evolutions provide evidence for element redistributions and phase changes in the surface layer of the SS420 plates. The element redistributions occur not only along the transverse direction in the individual laser-scanning tracks but also along the thickness direction between the surface layer of a few nanometers thick and the near-surface layer of a few micrometers thick while the phase structures are strongly associated with the spatial redistributed elements and their oxidations. The surface melting and its VL-dependent Marangoni convections in the melt pool are believed to redistribute the elements at elevated temperatures that, in turn, give rise to the phase evolutions of the surface oxides.