High-temperature oxidation behaviour of nanostructure surface layered austenitic stainless steel

The present study investigates the high-temperature oxidation behaviour of nanostructure surface layered AISI 304L stainless steel. A severely deformed layer of -300 mu m thickness, consisting of nanoscale grains (-40 nm size) in the topmost region, is successfully developed using the surface mechanical attrition treatment (SMAT) process. The SMATed layer is substantially stable up to 700 degrees C; however, the surface hardness is reduced by -37% at 800 degrees C for 25 h oxidation duration. Glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy analysis revealed the considerable difference in the chemistry and elemental distribution across the oxide scale of SMATed and non-SMATed specimens. Adherent, denser, and thinner scale, dominated by nanocrystals of Cr- and Mn-rich oxides, is formed on the SMATed steel. However, the Fe-oxide dominated scale containing micro-crystals is found on the non-SMATed specimens, which shows noticeable exfoliation. A high density of grain boundaries and lattice defects in the SMATed layer display admirable reactive diffusion properties of Cr and Mn during oxidation of steel, instigating the formation of a protective oxide scale. The SMATed specimens exhibit multiple zones in the oxide scale: (i) Cr/Mn depleted outer layer, (ii) Cr-/Mn-rich inner layer, and (iii) gradually decreasing Cr/Mn region.