Effects of surface microstructure on selective oxidation morphology and kinetics in N2 + 5%H2 atmosphere with variable dew point temperature

Abstract

Starting surface microstructure has been shown to influence selective oxidation morphology and kinetics on cold-rolled grades of CMnSi advanced high strength steels (AHSSs). The research presented below examined this phenomenon after 120 and 1 800 s and under N2 + 5%H2 with 0°C and −30°C dewpoint temperatures (DPTs). Starting microstructures were altered by pre-annealing, which led to decarburization and grain growth. All samples were oxidized in a high temperature confocal scanning laser microscope (CSLM) at 850°C. External oxides were subsequently examined using secondary electron scanning electron microscopy (SE SEM) and energy-dispersive spectroscopy (EDS). Cross-sections were produced and examined in a focused ion beam scanning electron microscope (FIB-SEM). Oxidizing atmosphere, surface microstructure, and time all influenced selective oxidation behavior. High and low DPT atmospheres had expected effects. Variation between microstructures was only apparent in 1 800 s samples oxidized in a high DPT atmosphere. Decarburized samples oxidized at higher DPTs exhibited surfaces covered with discrete iron nodules which may facilitate reactive zinc wetting.