Oxidation behavior of the directed energy deposited 316L stainless steel in supercritical carbon dioxide environment: Effect of post-manufacturing heat treatment

The oxidation behaviors of directed energy deposit (DED) 316L SS with various post-DED heat treatments (HTs) as well as commercial 316L SS were investigated in a 650 degree celsius/20 MPa sCO(2) environment. In addition to the detailed microstructure analyses, ultraviolet photoelectron spectroscopy (UPS) analysis was performed to measure the valence electron configuration, and the results were considered in understanding the observed oxidation behavior. Compared to commercial 316L SS, the unique large columnar grains and decrease in valence electron density of DED 316L SS benefit the oxidation resistance by retarding cation and anion migrations, even though they have similar oxide layer consisting of a Fe-rich outer oxide layer and a Cr- and Fe-rich internal oxidation zone. After stress-relieving HT at 850 degree celsius, the oxidation resistance is further enhanced owing to the HT-induced dislocation density decrease, resulting in a thinner sandwich-like oxide layer with chromia-type outer and inner sublayers and a silicon oxide middle sublayer. Additional improvement in the oxidation resistance is observed after homogenization HT at 1150 degrees C, with a thinner oxide layer and less inward intergranular oxidation, which could be attributed to an additional decrease in valence electron density.