Early Time Evolution of Selective Oxidation in a CMnSi AHSS

The influence of dew point temperature (DPT) and oxidation time on the selective oxidation of a Fe-0.09C-2.02Mn-0.91Si (wt pct) advanced high-strength steel during the early stages of oxidation was investigated in this study. Samples were annealed at 1123 K (850 °C) in a high-temperature confocal scanning laser microscope for 0, 30, 60, 90, and 120 seconds in an N₂ + 5 pct H₂ atmosphere with 243 K (− 30 °C) or 273 K (0 °C) DPT. Oxide morphology and chemistry were examined with respect to time and atmosphere DPT using scanning electron microscopy, scanning transmission electron microscopy, focused ion beam (FIB) milling, FIB serial sectioning, and energy dispersive spectroscopy techniques. Significant internal and external oxidation for all samples, including those with a 0 second isothermal hold, was observed. This indicated significant oxidation occurred upon heating. Samples oxidized in the 273 K (0 °C) DPT atmosphere exhibited intra- and intergranular internal oxidation and surfaces covered in discrete iron-rich nodules. These were attributed to stress generation during oxidation. Samples oxidized in the 243 K (− 30 °C) DPT atmosphere had shallow internal oxidation, sparse intragranular oxides, and surfaces rich in silicon and manganese containing oxide. Internal oxides for both atmospheres and all isothermal hold times were comprised of a silicon-rich core and manganese-rich outer shell was frequently observed. This core/shell structure was related to evolving oxide stability during the high-temperature exposure.