Surface texturing of steel by femtosecond laser and accompanying structure/phase transformations

•Surface textures of austenite steel were produced by femtosecond laser.•One-dimensional nanograting and microrelief of various roughness have been received.•Ferrite (α-Fe) in the near-surface layers was formed in spallation regime.•Phases α-Fe, Fe3O4, γ-Fe2O3 in the near-surface layers were formed in the phase explosion regimes.•Heat-affected zone was observed in the high-fluence mode for phase explosion regime. Topography, structure, and phase composition of surface layers of AISI 321 stainless steel textured by 1030-nm 320-fs-laser pulses were studied by scanning electron microscopy and X-ray diffraction analysis. Variation in single-pulse fluence and the number of pulses was found to change the laser-produced surface texture from one-dimensional quasi-periodic nanograting to microrelief of various roughness. It was shown that the nanograting formed in the spallation regime contained austenite, just as a non-irradiated steel surface, and ferrite. The redeposited oxide layers on the grating surface produced in the phase explosion regime with a thickness increasing versus single-pulse fluence were found to consist of austenite, ferrite, and Fe3O4. In case of high-fluence mode, the two-layer microporous oxide coating, consisting of Fe3O4 and Fe2O3 in addition to conventional phases, was detected. SEM studies showed that the submicrocrystalline structure of nano- and microsized ridges was preserved, except for the high-fluence mode.